Different design processes and NASA has to appease Congress who likes to cut funding if a rocket blows up.

But the Design-build-test-break-redesign-etc process that SpaceX uses is cheaper, quicker, and gives more data.

It took 8 Years AND $25 billions ($248 billions adjusted to today's dollar value).

For comparison NASA awarded a contract for spacex to develop the Human Landing System, the value of the contract is $2.89 billions.

Exploding rockets is totally common in rocket science. In fact, their mission objective wasn’t even for the rocket to succeed at making it to space. When you put millions of pounds of fuel into a tube and heat it up, there is a lot to take into account. No one has ever launched anything this big, so they are going to have to iterate quite a few times. Even the computer models can’t catch everything. Sometimes it is as stupid as a bad part manufacturer.

No, but the resources given and the requirements set are different. The Saturn V did not have to be reusable and was awarded two orders of magnitude more funding. Which is ultimately why it stopped being made.