why dont we say utcara?
Because they are already on the street. The only case when you should say "utcára" when you are going to leave your home.
Kimegyek az utcára. - I go out/down to the street
With "utcára", the street is the target. (On)to the street. But when you walk across, then the other side is the target. And that crossing action is expressed with the "-on"/"-en"/"-ön" endings.