Most are afraid to lose perks, some really don't have that many - veterans who have more then they need simply dont care about losing them.
Another key is most are rookies, they know if they leave the concrete they will simply be killed, so its kind of a two way street.
Its only perks, if you lose a few, then you farm them back, then lose them again....Vicious cycle, its called learning the game - if you don't lose them you will never learn to play fully, hiding on concrete will not teach you how to flank tanks or set up ambushes.