No one cares that they can get the same level gear. The point is that it creates an illusion that the player completed certain things, so you expect a certain level of play. If you set your PF for item level 105, shall we say, you would expect to get some people who know their way around and have beaten some of the harder fights.

But now, you have people joining groups that have only done some dungeons and hunts, know nothing about the more challenging fights, who join the parties expecting to learn or get carried when you were looking for seasoned experienced players. Then you waste your time finding out that they in fact have no clue what's going on and have to start all over again.

Everyone's time is valuable. Having peoples item level appropriate to the content they are clearing helps to ease finding the type of group you want. Having all item level accessible to anyone (example normal dungeons anyone can do and hunting aka whackamole) just totally defeats the purpose.