Also why they do non-standard stuff like accepting mail but then simply not delivering it, rather than bouncing it.
"Reputation scores" and blacklists / greylists started in late 1990s / early 2000s as multiple grassroots efforts when spam started to overwhelm systems. Gmail did not exist, outlook.com did not exist, it was easy and expected to run your own SMTP (which I did). And it was very unpleasant.
DMARC appeared 10 if not 15 years ago. There's no reason for a sane MTA configuration not to have it.
- it was a way to outsource the mail server administration and already free in our corporate package
Which is pretty much all I have from the corporate side. On the personal or small business side its most often
- I don't really care that much about running my own stuff
- I needed a personal email address
All these are valid reasons/motivations but none explicitly argue that email messaging should become proprietary.