Previous console generations had up to 16X more ram than their predecessors. This only changed now, at this very generation we are in, where we went from 8GB to 16GB so that's only 2X.
Think about it. We went from having 16X jumps to just 2X. If we continued the same pattern, the PS5 should have up to 128GB RAM.
PC users were spared this generation. Be glad.
Seriously though, this wasn't the reason, we just hit diminishing returns. PS4 to PS5 visuals aren't that different other than higher resolutions and some Ray Tracing added on top. Having noticeably better graphics requires more money, more work, better engines (lol) and other stuff that we can't have anymore, human game development has reached it's technological peak.
So we don't really even need more RAM or better hardware in general anymore, the only reason we are getting it is because next gen engines will be even more bloated and worse performing and newer devs even more used to having no restrains and care less about optimizing, so from now on newer hardware will be used mostly to brute force code that becomes worse and worse and graphics that become only marginally better at best..