Romanian does, in fact, have a reflex of Latin ambulāre: a umbla "to walk". (There are reflexes of ire as well, though it's become a future modal) I've often found that the familiar roots from other Romance languages do indeed exist in Romanian, though as in this case they'll have slightly different meanings.
The reasons for this are Romanian's relative isolation from other Romance languages, and its cultural domination by Slavic, Greek, and Hungarian neighbors for most of its history. Only Spanish, with its enormous number of Arabic borrowings, really approaches Romanian for depth and variety of non-Romance vocabulary.