So, there's several problems with that, but just the basic ones, which get more complicated when it gets to the details about the whole "how".

"An inside job" has a lot of ambivalence, and everyone can put into it their own interpretation. There are indeed some indications, that reports of something like it potentially happening were not properly followed up on, for example. And it's almost certain, that some of the Saudis that helped finance and support Al-Qaeda at that time also indirectly managed to gain money from the US - but there is no indication that any of it would have been on purpose for this. Both phenomena are very easily and fully explainable, by just miscalculations in Realpolitik and loss of control of a situation that was not taken to be that bad. The US was still riding high on feeling invulnerable after they became the sole Superpower with the dissolution of the USSR, so, underestimating threats makes sense there too. None of this would qualify for "inside job", in my opinion, but the term is ambivalent enough, that people can throw it around easily and move goalposts.

Then there is the logistics of maintaining und setting up a conspiracy like that. Iran-Contra, a much simpler conspiracy, was easily uncovered. Powell later lying about WMDs in Iraq was easily uncovered. Watergate, a super simple conspiracy in comparison, was uncovered. The sheer amount of people necessary to be in the know for setting up 9/11 as an inside job is ludicrous. It also gets the "Moon landing was fake" - problem. Just as with the Moon Landing, where the USSR would have had both means and reason to present irrefutable evidence, there were several international intelligence agencies, that could not have missed such an operation being set up, and would have had a motivation to weaken the US's standing in the world with irrefutable evidence. Investigative journalism could have produced at least solid evidence, yet all I have ever seen was really, really weak and not estimates shared by experts. (e.g. the whole "jet fuel steel beams" situation, where people simply ignored, that jet fuel, and other material, burns easily hot enough for steel beams to lose their structural integrity.)

Speaking of Powell lying about the WMD situation earlier: That clearly illustrates, that to further the imperialist agenda of the US, they would not have needed something so elaborate and risky. And oh boy, is something like this risky. Sure, there are no moral qualms with the US even back then before the even more blatant shifts that happened since. Especially US intelligence agencies would have had no issue killing US citizens, if they believe it to be in service of a "higher goal", e.g. imperialist influence expansion, tightening of surveillance and the bottom line of capital accumulation. But this is not just about the morality, this is about feasibility, necessity and the ability to control a developing situation. Because, no, there was no guarantee of this panning out the way it did, especially considering that if it really had been a conspiracy, proper leaks would have been almost inevitable with the amount of people you need to involve and sources you need to control and eyes that would be on the event (both journalists and hostile intelligence agencies).

The fact that they had to weirdly pivot to Iraq with said lies about WMDs and such, is also a good indicator of how unprepared but opportunistic they were about it. If planned out as an inside job, why not immediately choose to include more trails leading to Iraq instead of Saudi Arabia and Al Qaeda? Also: If planned as a pretence for tightening surveillance and starting a war, why do something so complicated and grand? Again, super risky. Why not just a redo of the WTC bombing in the 90s, maybe with a staged event and some people lured into it dead. That would be enough to whip people up if utilised with a prepared dis-info campaign on top. This way, they did a highly costly (not just in lives) move, that was all too complicated to do reliably without people finding out, and with a real risk that it could have shaken up the bottom line of people, too. All while scrambling to create an interpretation that suits their goals, instead of just creating an event, that already has that interpretation baked in.

The scale and weirdness about it is also, why so many truthers then add strange additional motivations, like "satanic human sacrifice" to the events, which I hope need no further refutation. Even the obscene enjoyment of "being able to break the rules" while in power as a psychological phenomenon, does not at all explain the overall dynamics at play - it's easy to see how that looks with the current president, or e.g. Saudi decadence of the ruling class.

So lastly, my question is: What is it, that makes you and others so emotionally invested, that it seems crazy to you, not to believe that it was an inside job? Because, I agree, they would have had no moral qualms, but to believe that, I don't need the event to have been an inside job, there is so much shit in actual history, that more than explains it. So why the focus on these grand narratives of conspiracy? The two main reasons I have found are: Fear of the actual chaotic nature of politics and history, where there is a genuine lack of control, even no control by some nefarious agents, or personal reasons, like disbelief about how friends and family gulped down the jingoism and nationalism Flavor Aid after the events, suddenly ignoring the fact that torture is now not only done in secret, but shamelessly discussed in the open, no mask necessary - and them suddenly accepting pretences for war, they would have at least been somewhat dubious about earlier. But for that to happen, it did not need to have been an inside job at all.

And my wall of text doesn't even touch on many other details of the more out-there stuff, like claiming "there were no planes" or other shit. All of which get more and more complicated and usually pretty wild in how they are attempted to be put into a conspiracy narrative.