Two reasons!
First, you can’t just load PC content on the Quest.
It isn’t that the Quest 3 can or can’t “handle” PC content – it’s that it is literally incapable of opening or using it!
When you package up an avatar or world, Unity has to know what system it’ll be running on. Depending on if it’s for PC or Android (Quest), it has to be packaged in different ways, with different layouts, with different programming built into it.
So, if you tried to load a PC world in a Quest 3, it’d just say “I don’t know what this is.” Same would happen if you tried to load a Quest world on a PC.
Think of it like loading a Playstation 4 disc into an XBOX 360. Sure, a game might have versions for both consoles, but you can’t interchange them. The company had to customize the game for each console, so they have different discs that don’t work in the other console.
It isn’t about performance, really – but, if you get into the technical weeds a little bit, the way the Quest’s graphics card (slightly inaccurate, the GPU is built into the same package that the CPU is) is quite different from PC graphics cards. They draw things in different ways that make them inherently incompatible.
I’m simplifying a lot, but that’s generally it for why you can’t just load a PC world on Quest.
Second: why don’t creators build for Quest in addition to PC?
It’s hard!
The Quest 3 is powerful, but it is still leaps and strides behind even the low end of modern desktop PCs. A good way to think about it is that laptops are about 4-5 years behind desktop PCs in power, and phones/mobile devices are usually about 8-10 years behind desktop PCs.
So, you have to do a ton of optimization to fit into that tiny little chip. Textures have to be greatly reduced in size. Worlds can only be about a quarter as large in actual physical size. Certain shaders and graphics simply don’t work (or cost way too much to use). Code has to be tightly optimized and carefully crafted. Not to mention you have to leave a ton of performance headroom for the terribly optimized avatars people tend to wear. 
That’s a lot of work; for some, they don’t want to do it and it doesn’t give them value. Maybe they’re just here to experiment on their own terms with the freedom that PC lends them, or maybe they just don’t really care to maximize the number of people visiting their world. Maybe their vision outgrew the Quest from the start. Maybe they don’t know any Quest users, and don’t really have a motivation to build for them. Or, most commonly – maybe they just don’t know how to optimize for it!
It’s a tough problem for everyone. It isn’t done out of malice.