Because the noise is so loud now that people are basically just coping with the stresses of work life by taking the easy path in the home life.

Because the protocol is flawed at its foundation - there is no good way to make sure that the person sending you something is a person and sending content you'd want to interact with. If the big providers weren't doing this stuff, we'd whine about the enormous amount of spam.

The solution is allow only communications that are demonstrably not spam and not unwanted.

There have been lots of proposals to this effect but maybe you have to put up two cents in a crypto wallet, which the other person can claim if the mail is unwanted. If someone takes your two cents, you remove them from your lists ASAP. The receiver doesn't benefit meaningfully from the 2c so they are unlikely to claim it. The sender is only damaged if a lot of different people claim the 2 cents. Maybe a few domains are default whitelisted even if they don't put up 2 cents, to protect against entities that are important but would be frequently spite-claimed (IRS, courts, and other similar). If you use a provider that doesn't whitelist, fine, but you're going to miss those emails.

Maybe the top 'spite claim' users are published and it becomes difficult to get people to agree to put up money to talk to you.

This reduces spam to the level of junk mail, which is still not totally trivial but is orders of magnitude more manageable.

A web forum's users, who love nothing more than using the "flag" button to stomp out comments they don't like, are indignant upon discovering the world's second largest email provider has a "flag" button of their own.

I don't disagree, but what are your options when a giant portion of the population uses O365 and outlook.com? How are you going to get them to stop using it, so that legit emails from user-owned servers are not silently rejected?

Most of them wouldn't even care probably...

That may be so, but you can't control what mail service your recipients use. So whether or not it's a bad idea, it can be your problem. That said I don't agree with OP that self-hosting mail is a bad idea.

How often would you be able to detect acceptance-but-non-delivery of your emails to office.com-hosted emails?

It's not enough. I have a domain that I had a very long time but it is on an undesirable tld. I got it long before it was problematic. Outlook always puts my mail in the SPAM folder even though I have done everything I could. I even deliver through a trusted provider. Not on any spam list. DMARC, DKIM, SPF all set up. No dice.

I've never had a problem not having dmarc. DMARC doesn't really mean anything. The security comes from DKIM (or SPF, but that's garbage)