Yes for a 1-1 it isn't that rare. Obviously for someone that wasn't picked 1-1 it is very rare.
Arod, Griffey, Upton, Harper (he was as much HS as college). Even Correra was in the majors at 20 (he was 17 when drafted),
Yes it's a select group but you don't see a ton of HS bats for 1-1.
You see it with lower picks as well. Trout of course. Heyward was in the majors for the whole season at 20.
Sure it's rare even for a 1-1 but these are normally the highest ceiling prospects.