Set up dkim/SPF properly, make sure the ip you plan to use is clean before you start, sign up for MXtoolbox blacklist alerts and if you get on a blacklist (doesn't happen often if you do a bare minimum of proactive security), you request removal. It's really not hard.

I am running my own mailsever for over 10 years without any blacklisting problems...

I work as a Sysadmin for a web host who sells VPS's. I've helped many people setup domains on their server to cover SPF, DKIM and DMARC passes on a daily basis. Most use these for personal or business level mail delivery without issue.

Are there hurdles to overcome? Sure. But it's not exactly hard as long as you have a IP that's isnt a poor reputation (which as an ISP we help delist and improve). But it's not impossible.

Its more "convenient" to use a third party mail provider just as Office365 since you pass on all that setup and responsibility onto their framework, but it's not hard to setup a decent level of mail service yourself.

have mine running for nearly 20 years now and never had any major issues with delivery 🤷

I've never had any issues getting mail delivered to major providers

That's not why people move to big hosters.

They move because you don't need to waste money managing them, and they have reliable backup

We used to host our own, but big providers are so cheap and have such a good interface that it doesn't make sense to host our own.

It's the same reason why most companies don't host their own web servers.

Even large corporations use AWS or similar.

You're spot on, and even smaller ISPs routinely get blocked by larger hosters (anyone who doubts this, please look around for the many stories along the lines of "gmail silently drops my email")

Residential IP blocks are scored much higher and given a negative trust from the start - not surprising since that's where much of the world's spam comes from through compromised computers, routers etc.