Because we can't have nice things? is there any answer other than that
honestly that is my number 1 complaint of this entire site half the posts are either started by or responded to by people with the premise of "this is way too powerful i can't believe this is allowed or worded this way how can i make this more realistic and lower the power level in my game"
honestly my answer to all the cheese people is if you don't like it go find a new freaking game and stop bothering the people simply trying to have fun
the other option is start the new players at 6000 gold or have them each start with a +1 weapon and armor on top of WBL so you don't completely make what the existing players did meaningless, but you also keep the new players from being too far behind
you are the DM remember everything in the books are guidelines and the goal is for everyone to have fun