I was content to let the other comments address the history since I’m not particularly well versed there (and there’s already enough confidently incorrect bullshit in the world). I mostly just wanted to interject on why there aren’t more chip companies beyond just hand waving it away as “market consolidation”, which is true, but doesn’t take into account that barrier for entry in the space is less on the scale of opening up a sandwich restaurant or boutique clothing store and more on the order of waking up tomorrow and deciding to compete with your local power/ water utility provider.

The answer also gets kind of fuzzy outside the conventional computer space and where single board/ System On a Chip designs are common, stuff like Raspberry Pi’s or smart phones, since they technically have graphics modules designed be companies like Snapdragon or MediaTek. It’s also worth noting that computers have gotten orders of magnitude more complicated compared to the era of starting a tech company in your garage.

If it helps answer your question, according to Wikipedia, most of the other GPU companies have either been acquired or gone bankrupt.