Why is Switch selling so much, yet PS5 and Xbox Series X is selling so little?

One main reason, at this point, PS5 and Xbox are still a bit supply constrained, although not as bad as last year.

Can't buy ps4 or series X here. Question should be, why are there so few ps5 and series X getting made.

Demand for the new consoles, especially PS5 remains extremely high but stock has serious issues.

but they are doing extremely well despite issues and high price. 

Series X and PS5 are completely sold out.
Nothing available online and whenever the news pops up that a store has a few of them, people are racing each other to get there and the consoles are sold out in an hour or two.

Series S is somewhat available (at least here in Germany).

I dont know whats going on, but I think Nintendo is very happy with the situation.

Global silicon shortage affecting a ton of new products. From Consoles to PC components to Cars. The switch has had tons of stock while the ps5/series X are new products. I'd imagine if the Switch pro is real and it comes out with new tech during the shortage, that will also be hard to buy.

Because Switch is at it's peak, while the other two are new. The shoe was on the other foot as the Switch released, PS4 clearly dominated Switch, now it looks like Switch will outsell PS4 lifetime.

I am amazed how many here answer with stock shortages. Realistically the Switch would still dominate if plenty of stock were available. Because no fresh released console beats a successful console at it's peak.

I wouldn't say they are pointless, Some people do want it but the majority wants the X. Just like some people want the switch light but more want the full switch. Microsoft just miscalculated the  demand split. 

Because launch sales for any console will never be able to beat a different console within its prime (provided said console is also successful).

Even if stock was not an issue, Switch would still be out selling them.