because amateurs can't stand to maintain tension in a given position...it is a hard habit to form--maintaining tension
Edit: And also because leaving pawns in the middle tends to produce more positionally oriented games whereas breaking the tension in the center tends to allow for more tactics and most amateurs love to describe themselves as "tactical players" rather than "positional players".
Yeah, kingwraith pretty much said it. Many weaker players tend to be obsessed with either exchanging, or pushing ahead of an opposing pawn (especially if it gains tempo! 
) when there is pawn tension.
They just don't understand why they should maintain that tension (till the ideal time to break it) a lot of times.
A very strong IM I used to be friends with used to love to use the phrase " tension retention ! " a lot and when I once questioned him about this he told me that the tension in a position should NOT be released/resolved unless it brings one advantage OR avoids a clear disadvantage. This advice has always stayed with me in positions with tension in them.
As long as you mean weak as in sub-master players, then yes, they tend to release it too early. Beginners... well they do it the first move they get, us "sophisticated" higher class players pride ourselves on leaving it for... gasp 2 or 3 moves... but still tend to release it too early or at the wrong time.
3 other good posts with good info for sure -- but I appreciate Reb's rule of thumb about keeping it unless you'll clearly add an advantage because of it, or will be worse otherwise. I'll remember that.
Hi, newbie to Chess.com. (great site) Used to play a lot 30+ years ago. Retired now and getting back into Chess. My question is: Why is an early pawn exchange in the center of the board so common?