A program is only allowed to graduate a certain number of people per year, and they must have fulfilled all requirements. You can't put a pgy2 into a pgy3 spot because they will be "skipping" a year, which is not legal. While hiring another resident to the program keeps the total number of residents the same, it is the number graduating per year that is limited...your program would need to get special permission from the ACGME and, if approved, would mandate an RRC site visit to evaluate the program to ensure that it can accommodate an extra resident at that level based on the program's available resources. In other words, it gets rather complicated.
Actually, this is not quite correct. Programs are limited by their ACGME cap, which limits the total number of residents. If I have a cap of 30, I would usually have 10 residents at each of the 3 PGY levels. However, nothing stops me from having 30 PGY-3's if I want to (except for the havoc it would create with the schedule, having no interns).
There is no rule about the maximum number of residents graduating from a program in any year (although if I actially did have 30 PGY-3 residents, I expect my RRC would not be amused).
However, let's say I lose a PGY-3 so now I have 10+10+9=29. I can certainly take you as a PGY-2 -- now I have 10+11+9=30. All is good, right?
Wrong. Next year, when I go to the match for 10 more interns, I end up with 10+10+11=31 residents, and that can't be without special permission. So, if I take a PGY-2 into a slot vacated by a PGY-3, and then no one else leaves the program, I might have to decrease my PGY-1 count the next year to compensate. Then I'm short a PGY-2 the year after, etc.
So, many programs avoid this to avoid the crazy accounting that occurs in future years.