Why is Nintendo in particular this aggressive against emulators? Why haven't we heard of Sony going after PCSX2 and RPCS3, or Microsoft going after Xenia and Xemu?
It is likely due to age and popularity. At this point PCSX2 is the only widely popular emulator you've listed, but is older, in the same way Dolphin is old and less of a concern of harming their limelight. That isn't to say those other emulators aren't making waves lately, but there is a window between age and popularity that makes this software a prime target. Especially emulators for current gen, which has been historically very rare, and when it did exist were no where near as good as CEMU, Yuzu, and Ryujinx were.
I think this also shows how underpowered Nintendo consoles are; the Switch is essentially a slightly upgraded Wii U, which itself was only marginally better than the PS3 and Xbox 360! If Nintendo really wanted to combat piracy, all they had to do was beef up their flagship console to the point where it's hard to emulate.
Probably for the same reason Nintendo shuttered the stores for 3DS and Wii U over the last year. They are attempting to consolidate infrastructure, support, user base, and sales.
Nintendo's infamously protective of its IP, even to its obvious detriment. I know a lot of people who jumped on the nomoretendo bandwagon after AM2R got DMCA'd, because the smart and morally correct thing to do would have been to hire that person and pull his incredible talent into the next Metroid game, because they clearly had a lot of love and respect for the subject matter and the technical expertise to put it into action. It's hard to overstate just how fucking good and polished that project was, virtually identical to the Fusion/Zero Mission engine, except in the ways it was more improved. It was certainly better than anything Nintendo had done with Metroid in quite a while.
Because none of those emulate current generation systems.
People act weirdly entitled about Nintendo product, throw shitfits when Nintendo reacts exactly like Sony or Microsoft would to them doing the same thing, then act extra hurt that Nintendo did it.
According to what i've read around, the Switch 2 will be retro-compatible with the Switch and a big sellout will be possibility run all the old games in full hd with higher frames (something emulators already do better). Also this may have been used to adapt current emulators to the new console games.