Why always go up or down?
上 as a verb could mean "go to" (i.e. could be replaced with 去 without change in meanings) and "ascend/board" which could be understand as go up onto since the floor of vehicles are generally higher and require a upward movement. Anyway, in phrases such as 上卧室, 上 just means "go to". In 上厕所, 上 could mean "go to", but more often it contains the meaning of actually using the toilet. So it is possible for a person 去厕所 but not 上厕所, but most of the time such distinction wouldn't be made.

As for 下 as a verb, it means "go down to" physically or figuratively. In 下厨房, 下 contains a figurative meaning of "get down to the work", so if you are just getting something from the kitchen you wouldn't say 下厨房 and you could say 下厨房 for meaning of "go to the kitchen and cook".

上 and 下 are also used to distinct the start and end of certain activity, such as 上课、下课、上班、下班. In such scenarios, 上X could also be used for the time points during such activity.

Anyway, these two are pretty hard to master via a clear set of rules. You're just gonna have to get familiar with as many patterns as possible.