Well, you need ~4 times the power to render the same thing in double the resolution, let's say you're rendering things 4 times the resolution (720p to 2160p) so it's a divide by 8 factor and you're already down to it being in reality ~6x more powerful.
Add to that techniques that require extra passes like SSAO and post processing and it's clear why games are not 2160p native, this is in order to reach a bigger differential against prior consoles.
Bottlenecks have reduced, memory increased a lot (if you compare textures, PS3/X360 vs PS5/XBX the gap is comparable to N64 vs PS3/X360) and the efficiency of flops have increased which is a godsend, but native "pixels" remain the most expensive computational result you can hope to achieve.
Of course though, optimization also plays a part, the more power you have the less you have to optimize, so a lot of optimizations is left undone in modern hardware, that can be a good thing in a multiplatform sense. In the past plenty of games were tied to PS2 and PS3 due to extreme optimization. That has no place to exist in the present.
I think people who dwell on the "50x better" statement of the OP are missing the point here. I was watching DF's Horizon Dawn FW PS5 vs PS4 comparison video last night and I completely get what OP is trying to see. I don't know if it is diminishing returns or the evolution of game engines and tools with scalability but I can't think of an example of a PS2 game scaled back to PS1 but still looking almost as good and without cutting back or altering the actual gameplay.
That would be hard because PSone had such a primitive 3D engine.
Even in the case of Dreamcast vs PSone and the content being mostly the same (Rayman 2, for instance) felt like a generation apart just because of image quality.
To be fair, the difference from Xbox 360 to PS4 in image quality is kinda huge in my book even if when it comes to it still both are "HD consoles". It's not lack of pixel, it's color depth and motion (framerate, framepacing and no stuttering/vsync issues too).