why sell the company instead of enshittifying your platform yourself?
Because it's a lot easier to find someone who thinks they can do it than it is to actually successfully do it yourself - as we are currently seeing with how wonderfully incompetent Spez is with Reddit.
When Yahoo bought Tumblr for $1.1 billion in 2013 - only to sell it for $3 million in 2019 - was Tumblr bringing in millions and millions of profit? No. But Yahoo thought that they would be able to make it.
Elon Musk paid $44 billion for Twitter, it hasn't turned any profit either (and never will enough for him to get his moneys worth, but that's just because Musk is an idiot).
But yeah, quite often it does feel like a scam. Or kinda like... gambling? You hope someone will pay a lot for your company, while they hope they can make it turn wildly profitable, both may or may not come true.