
Because most of them are underage... (Or under the legal drinking age) And therefore they get drunk pretty fast....
The more you drink, the more 'tolerant' you get, but only very few people have high alcohol tolerance from the beginning....
(I am actually more amazed about the fast recoveries... And the fact that there are very few hangovers.... XD)

Well majority of the uke population have a low tendancy to alchol but there are some that dont so you really cant say every uke.
e.g) http://www.mangago.zone/read-manga/datte_maou_sama_wa_kare_ga_kirai/
Why is every uke so unbelievably intolerant to alcohol?