This happens because to enter a new game you must put money in the pot blindly. Also, if you don't post behind, you will also start with bad position. It takes a while to get a hand you will then play and when you do, it's not like you win every hand you play. You start out behind.

The other reason is that most good players start out extra tight until they have a feel for their table and then they can decide how they want to play. You may pay a few blinds before you really start playing.