To be fair, it's plausible. They might not have wanted a home inspector writing up "low water pressure" as a potential problem. 'Course, the inspector might write "water splashes out of the sink" as a problem instead, but that at least is more straightforward to solve, rather than being possibly indicative of a bigger hidden problem.

They obviously didn't solve it, since it still exists. In this scenario they put a bandaid on it and then when selling put it back to normal so the next person can do what they please.
Or they could have thought it would be a funny prank.

I think that's much more likely than them completely braindead and suffering the whole time, like you think. That's certainly possible too, though.