??? there are android emulators that can emulate the switch. now compare your phone with the steam deck and ask yourself if your phone has the same power than the steam deck. doubt it.
i think you seriously underestimate the steam deck hardware and what difference parts make that have been developed more recently, even if the numbers (of cores for example) are not impressive.
Also its not just raw "power" that makes one system emulate another well. When emulating a console like NES, N64 or anything else from that era, the host systems never had to care much about optimization because the jumps in "power/performance" are just extremly huge and the operations of said console do not require "much". We now live in a time where developing, just like the hardware itself, gets more refined and we try to get every drop of juice out of it. it makes a huge difference. those claims of "you need powerfull hardware" are still true but are from a time when pcs couldnt emulate xbox 360's. that time is long gone.