why is it aceptable in frum comunitys to wear tight sleeves all the way up to the shoulders if it is forbiden to do so for the legs?

Shalom, The reason for the differences between arms and legs in the laws of covering stems from the fact that the legs require additional levels of modesty as they are close(r) to the (more) private parts of a woman than are the arms. (See Benay Banim, Vol.3, 26). Blessings, D. Sperling.