Tbolt, Psychic and Surf do not cover all of the bases, though. You absolutely need Focus Blast to immediately punch through Ferrothorn and Tyranitar for Raichu's teammates, and Thunder / Psychic and Surf are obviously non-negotiable. Nasty Plot's usefulness is limited; it can do well against stall, but it's dead weight against non-stall teams (which currently make up the majority of the metagame.)
Of course they don't cover all bases, that's why you have a whole
team to support your Raichu - and while they don't cover all bases, the blind spots are pretty minimal. Besides, a LO Electric Terrain TBolt still does well against an uninvested TTar (which I think is most of them currently, they tend to be running offence):
252+ SpA Life Orb Raichu-Alola Thunderbolt vs. 0 HP / 0 SpD Tyranitar: 169-200 (49.5 - 58.6%) -- 99.6% chance to 2HKO
I think I would rather run something with an appropriate fighting move for TTar and Ferrothorn rather than take such a risk on Focus Blast. I appreciate stall is less popular now due to the raging whirlwind of offence we've got going on currently, but it won't always be the case and I think being able to get through walls on both sides of the spectrum definitely gives you an easier time of things that packing a shaky coverage move.
I've been running A-Raichu and, while I'm not the most experience player, I can definitely say that A-Raichu just has too many issues to set-up on it's own with Nasty Plot. 60/50/80 Def is not hard to get past for any team. I agree it's a huge surprise factor for Raichu and if done successfully could net a sweep, but there are just way too many things that would need to go in your favor + pretty spot on prediction to boot. The NP set is specific for Koko teams meaning you have 5 turns for ET, the first of which ticks when you do your switch meaning there are only 4-turns in which to use A-Raichu. A-Marowak is the best Koko counter and isn't getting OHKO'd by any move that Raichu has(even Surf is a 2HKO). But then let's look at a rain team where you will more than likely see A-Raichu being used. You now need rain down, koko needs to have gone out and then you can use your Raichu. But - Only if you've dealt with Raichu's counters already or he's wasted. As well as dealing with any priority as 50 Def won't take much. You need to do a lot of things to get Raichu out in an optimal environment already and then you want to burn a turn on set-up with NP? Unless I'm missing something huge(and I do understand priority is rare, but it still exists) I think that's why people have an issue with the NP set. It's a lot of set-up to lock yourself into a 1-1 most of the time.
I'm actually using A-Raichu for it's Z-Crystal and have it set it's own terrain. Which I know is pretty frowned upon, but the point is that any type of set-up leaves Raichu open and vulnerable. The only reason I even have the ability to do that regularly is because I use A-Ninetales with AV allowing me to leave Raichu in against mons that should be able to OHKO/2HKO. And to be completely fair there are a lot of pokemon in the meta that can quickly OHKO/2HKO Raichu.
Well I've also run NP, and have found situations to set it up, so go figure. You don't need to set it up every game as I mentioned, that's not really the idea of the set, it just means you can wall-break when you need to. It's very easy to force a lot of switches with this thing as well, so a Nasty Plot can be produced fairly easily - and then you can't really wall it easily and the speed is extremely good. As I've said before, Raichu is already fast - so I don't really care about burning through a turn of something that makes it even faster (the electric STAB hitting harder being a null argument as NP makes it hit harder than it ordinarily would anyway).
But I get it, people love the crazy offence sets. They certainly are the easiest sets to run, so I can see why.