Why is there limited theory?

The number of variations.

 The opening analysis can only predict so much. Let's say both players play the exact moves of a main line, now you should be in a playable, roughly equal, middle game. And from here (in a roughly equal position) strategic plans should be drawn up to strive for a positional advantage. The chess opening is not intended to lead you to a totally winning position from move one to checkmate.  A lot of players tend to over study the chess opening thinking it will lead them to victory, but you have to be able to afterward play a strong middle, and endgame!  More info 

 It's unlimited because the more moves you have in a game the more variations start poping up. Eventually you reach a point where there are too many variations and not enough people analyzing them. Welcome to the infinite world of possibilities called "chess"! x)

You are CLUELESS. There is unlimited theory. You can't know all the existing theory. If one could memorize all that, he would win most games ( about %70 ) against lower-than-2200 players. Off topic: @Yigor I created a team called Stockfish Stems on lichess. I make this site's ( chess.com )SF play against itself and post the games. Some interesting games are played. You can join if you have an account on that site.

Databases are a good clue to the situation: At some point the finite number of games played is balanced against the infinite number of possibilities, so the database tree of moves ends there, with no more data available. That point is often at about move 20, but it varies considerably, like from move 2 through move 30. There is no reason you cannot push beyond the database end. I do that all the time since I have played the same program (Chess Titans) for years and collected most of the limited variations it plays, so there is an extra amount of data available that way, especially for unusual lines that have limited database coverage, so some of my own repertoire lines start at around move 30.

"... I feel that the main reasons to buy an opening book are to give a good overview of the opening, and to explain general plans and ideas. ..." - GM John Nunn (2006)
"... the average player only needs to know a limited amount about the openings he plays. Providing he understands the main aims of the opening, a few typical plans and a handful of basic variations, that is enough. ..." - FM Steve Giddins (2008)
"The way I suggest you study this book is to play through the main games once, relatively quickly, and then start playing the variation in actual games. Playing an opening in real games is of vital importance - without this kind of live practice it is impossible to get a 'feel' for the kind of game it leads to. There is time enough later for involvement with the details, after playing your games it is good to look up the line." - GM Nigel Davies (2005)

The following position offers a lot of counterplay: