Why not just do away with third-party cookies altogether already?
Another example is you're signed into website A, and while on website B, iframes to website A behave in such a way that you're not signed in, and you cannot sign in.
This is because the download is from googleusercontent.com while your browser remains at drive.google.com the whole time - and to download private files, googleusercontent.com expects you to have a login cookie. If you block third-party cookies the download gets stuck in a redirect loop, sending you to get a cookie over and over again.
Google is aware of this but hasn't fixed it.
1) One of my local banks (who use weird third-party hosted modules for some of their functionality).
2) Verizon.
3) T-Mobile
If I were a normal user, any one of these ("I have to do _what_ to see my FIOS bill?") could be a show-stopper.
Which is what makes it hard to turn this on by default without driving away users.
This thread contains plenty of examples of legitimate uses for third-party cookies. If FF instantly and immediately broke those, users would be cursing, not praising Firefox, and switching to a browser that doesn't break what they use.
Historically getting some browsers on board with that program has been very difficult.
Concretely: a large fraction of website developers would much rather put up "only works in Chrome" notices than fix their crap.
[Disclaimer: I used to work at Mozilla, and have done my share of trying to push for turning off third-party cookies.]
It's google. I'm talking about google.
Not on iOS
A lot of websites depends on this via auth0, cloud identity, cognito... and the experience becomes subtly broken in a way that you need to be extremely technically savvy (a developer that has a whole lot of auth experience) to understand.