[Sanzig]

I think you nailed it.

Somebody trying to earn some money from their writing - yeah, I get why they choose Medium. It sucks, but there's not really a tremendous amount of better options.

But the majority of Medium articles aren't that. They're promotional personal blogs (so why not just put those on your personal site?) or more often they're technical blogs written as marketing material for a company. That can easily be hosted on a company's own site with little effort.