Other vendors are free to produce compatible parts. They publish physical dimensions and cad files on github.
Everything about this is as interoperable as possible, both physical and software.
Maybe no other vendor will produce a motherboard or keyboard, but it's not Framework's fault.
Second, The closest competing product to Framework is Lenovo not HP. (Despite the fact they look like a Mac's aluminum body, huge buttonless touchpad and black chicklet keys, with a Surface's screen aspect ratio.)
HP's customer is someone who would like a Surface or Mac but doesn't have that kind of cash, or just cares more about a distinctive look that isn't gamer.
Here's HP's customer: I got my mom a top of the line maxxed out HP because she will never care about upgrades or repairs or Linux or raw power, but she does care about the blingy rich bronze look, and I care enough to steer her away from Surface and Mac even though I don't care about the cost. That's HP's customer.
I WAS actually able to replace the battery in her previous Spectre (the sweet thin one with the funky hinges that looked like hoop earrings or wedding bands) to give it to my niece when I updated mom, but HP did not make that easy.
HP are premium-looking Chromebooks that run Windows.
Framework are user-serviceable open platform Lenovos.
"Why would I spend..."
You clearly wouldn't, so don't. But I would. Why? goes like this:
I don't particularly care too much about AMD vs Intel, but a lot of people are asking for an AMD cpu motherboard.
Let's say Framework did not make a an AMD motherboard, but someome else did. Let's say that the only way to get a Framework was to either buy a whole Framework including an Intel mainboard I don't want, PLUS the 3rd party motherboard for $500 or whatever it is. I would rather do that, because I want that open platform. First, Framework would not make me buy the entire machine, they would let me buy everything but the main oard. But even if they didn't, that mainboard I didn't want is actually useable all by itself like a 900 horsepower raspberry pi. Or I could sell it, because it's useful to anyone else too. Or I could keep it as a backup in case I damage my prefferred board. That platform which makes all kinds of options possible, is valuable to me.
No one yet makes any such 3rd party mainboard, but the platform at least allows for it and makes it possible vs not-possible. I want that. That is valuable to me. I will pay a lot for that.