Queen's Gambit blocking in the bishop

I'm learning to play the queen's gambit now, and I was wondering why in games it is better to play e3 before getting the dark square bishop out.  Isn't it more active on f4 or g5?  Why do masters choose to block it in?

well this is the queens gambit accepted. and white here needs to make sure he puts pressure on the c4 pawn right away in order to make black's defense of it too awkward to bother. also black will quickly play c5 often and white will be thankful he stabilized the center first. 

5...Nd5 6.Be5, (6.Bxc4, Nxf4 7.exf4, Bd6 8.Ne5, o-o 9.Qb3, Nd7 10.o-o, Nb6 11.Re1 =/+ I found this better than 6.Be5) b5 7.e4, Nf6 8.Nc3, Bb4 9.Nd2, a6 10.a4, Bb7 -/+   ....so says Fritz, but my comp is a decade old.

I think the answer is that after the Bishop move Black gets the better position.  Some of the variations that I saw are a bit complicated and for lower rated players its probably playable.

Thanks blasterdragon.  That helps a lot.

I think 6.Bg5 was bad and causes the very problem that resulted on whites Kingside.  Better was 6.Bxc4, Nxf4 leading the parenthetical variation I gave, where Black is equal or only slightly better.

For beginners, it may not be a problem but the Isolated-d pawn and the double pawns are a great liability on the white player, hence Bf4 is not a common choice.

Real simple reason why. The board is open. And one way or another the Bishop will hurt white if it is not blocked in. But it does not have to remain that way. And just because it is blocked in does not mean it has to be a bad thing.

Understand the opening? Ok Understand this variation? Your confused. Your goal is get back that pawn as White is down a pawn after 2.... dxc4. And in some cases that is all that is needed to win a game for black. white needs e3 to have the f1 Bishop attack Black's c4 pawn as soon as possible. So the c1 Bishop is going to be blocked in and serve as a defensive piece at the moment.

I agree, but the comp does not think its completely losing.  After 7.exf4, Bd6 8.Ne5, o-o 9.Qb3, Nd7 10.o-o, Nb6 11.Re1 how does Black play against the weaknesses of isolated pawn and doubled pawns on the f-file?

Although, I do have a doubt so as to why we can't play:

How about this as a tip:
Don't play Queen's Gambit :) 

i think the reason is because

no problem when i first played the queens gambit i had the same question as to why but then after actually trying to play it i got in a really tough position

yeah but i think its a bit harder to play white in that position granted white does have a lead in development but whites pawns are doubled and black has the bishop pair so long term black is easily equalised or better

3. e4 is one of the main lines, as are 3. Nf3 and 3. e3.

The reason white can block in the bishop is that black's bishop is blocked in too, and black's only real active plan is to play c5 and eventually cxd4, opening the white bishop. Otherwise white can just slowly expand and gain a nice advantage.

The e3 line isn't played as much anymore because of e5 when the isolated pawn position is not that much better for white (black has opened his bishop and has some other plusses over the Nf3 line)