This is not really the case. If you sign an 'old-time' vet for a 1 yr. vet minimum salary, you get a substantial break on the cap cost. This change was implemented a few yrs ago, 'cuz there were many back-up vets (AVP comes to mind), that couldn't get work for the very reason you state. The extra 400k or so of cap room that they took up wasn't worth it relative to a rookie who can sit on the bench for much less.
As a representation, (I don't know accurate #s, but this'll be the general idea), let's say that vet min for someone w/ 10+ yrs is $900k but their cap hit will only be $500k. Rookie min is like $250k, so isn't a good kicker worth $250k? You'd be better off w/ only 52 guys on the roster.
(My personal opinion is that kickers (& punters) are among the most valuable players on the team & getting a good one, like Moorman, is worth WAY more than there relative cap hit relative to other players.)