Currently, because MS and Google more or less have a duopoly on email. They don't trust you not to send spam, and they will make it as hard as possible for you

Currently, because MS and Google more or less have a duopoly on email. They don't trust you not to send spam, and they will make it as hard as possible for you to prove that you're not a spammer. Ostensibly this is so they can protect their users. More cynically, I suspect they're quite happy about this. Historically, because we are - for whatever reason - unwilling or unable to deal with spammers. I mean the people sending the spam and profiting from it. There are virtually no repercussions for spamming millions of people with garbage on a daily basis. Every cent you make is profit. Putting spammers in prison would make it a lot easier to go back to hosting our own mail servers.