How did PCs beat out the Amiga, Mac and ST with nonsense like that?

I think you can ultimately blame Compaq. It was the first “pc clone” that showed the market that a PC not from expensive IBM was viable. After that even if you weren’t buying a Compaq your own generic clone was “good enough”. So You could access hardware and software built for a $4000 8088 IBM PC with your $1200 clone.

Amiga never was commodity hardware. It was always expensive. It didn’t get cheap enough fast enough. Amiga 500 came too late.