In part because low end chips are not needed, as they can be had more efficiently as part of the CPU for both AMD and Intel. And since these are the only 2 options for X86, Now that VIA has discontinued their X86 line acquired from Cyrix, there is no low end entry point for a new maker of GPU.
The natural evolution is to start from a lower end, and if successful work up. This is not possible in the PC market, and makes entry to the market near impossible, except with enormous investments that may never pay off, especially since PC is a dwindling market.

As you mention Intel is dipping their toes, but despite doing a pretty good and big effort, and investing a lot to develop a better GPU, and actually delivering a good product at a reasonable price, that should be absolutely competitive on paper, their marketshare is absolutely minuscule, because Nvidia and AMD together dominate and already fill the needs for the mid to higher end market, and have brand recognition for graphics.

It’s not that there aren’t technologies that possibly could compete if scaled for PC, because those are actually pretty numerous on phones and tablets. But you can’t port these cheaply to PC, because there is no market segment for them to slide in to easily.
It would require major investments to make them actually hardware performance competitive at higher scales, and investments in making good drivers. Intel had a big head start in these aspects, already making on-chip graphics that had drivers already. And still they are struggling, despite delivering a good product, and people have been screaming for a third option,because of high GPU prices.

This may not be the entire explanation, but I think it’s a very significant part of it.
The better question IMO is why Intel never became more popular, considering how much people have raved that more competition in the GPU market is required.

And the explanation for that is:

but let’s be real, AMD and nVidia are the only options

Except Intel actually presented a good alternative, but was never seriously considered by most people for whatever reason.

Personally I didn’t consider Intel, because I remember earlier attempts by Intel, where Intel quickly left the market again. And I didn’t want a GPU where I’m left without driver support a year after I purchased it.
So in my case it was lack of trust in Intel to stay the course. But every other maker would have that exact same issue.
There have been a few attempts in the past from other makers, but they all had performance or driver issues or both. Intel delivered a stellar product by comparison. And if Intel drops out of GPU again, I think there’s a pretty big risk it may be our last chance for a third significant mid-high end GPU maker on PC.