Its not a factor of processing ability. Any 3 yr old $600 PC with dedicated graphics has more processing power then a ps4 or xbox one right now.
It is a mater of figuring out every proprietary format and coding on the system that allows the games to play. Sony and Microsoft don't just give that away once they release a new model (which is likely based on a large amount of the proprietary code.
Emulators are made from people that was able to completely decode the gaming platform either because the OEM did not make it difficult at all (remember sega dreamcast), or because the console is so old that the OEM has nothing to loose (nes, snes, Gameboy, etc).
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Not to mention the simple fact that any console from a single generation ago still has patents and copyright on that code so any emulator would be completely illegal.