It's cheaper. Many fonts require ongoing licensing fees based on views or some other metric. Creating your own font means you own it — no more fees.

Part of it has to do with licensing costs but also "brand-able" ownership.

Say if you use Arial or Helvetica for your logo, then it's a generic typeface easily reproduced by whoever else that has them installed on their computer. So often, many brands for their logo take a generic font and customize it to make it their own. However, when you customize a font for a logo the font file itself is not customized, just the vector version of it for the character of the brand name. So if you want to extend the usage of the font style to say headline text or advertising text, then you need a whole custom built font. Custom fonts cost money, but picking a pre-existing "designed" typeface from other type foundries cost a lot more than getting a custom font. So two birds with one stone, you get a unique font for your brand and don't pay additional licensing since you own it.

Licensing costs. It’s cheaper to design your own typeface than to license others when a platform reaches a large scale.

I didn't either, until I tried typing a capital F, followed by a period, followed by a (regular, non-curly) quotation mark, on Twitter, and saw how it condensed it down far too much into an ugly mess—"oh, designers just have too much time on their hands."

I'm curious about it too, I'd love to read something by someone with more specific insight than I have.

I know that people love to make fonts: it's a really common hobby activity and people make incredible fonts all the time just for the hell of it. I don't really know how that translates so commonly into the institutional will to do it that every company seems to have. I can kinda guess but it's just conjecture.

Hipster design teams. Typography lets them be "artists", returning to bohemian roots. In reality, they're just designing apps around dark patterns or whatever the product management team thinks is righteous. Companies can't say no, they're in demand, so its really a "20% project".