Despite the fact that both ходить and ехать are imperfective verbs, the verbs formed by adding prefixes to ходить are imperfective, while those formed by adding prefixes to ехать are perfective.
Question 2: Why doesn't ходить form perfective verbs when adding prefixes, with the imperfective forms created by adding the same prefix to the iterative/frequentive form of ходить, хаживать?
(It gets even more confusing if one also compares how verbs are formed for возить/везти/носить/нести when adding prefixes, but I will stop here for now.)