Why don't we play different openings in chess? Chess comes in many forms, both with respect to the clock and the board format. 

But why don't we have chess tournaments where we play different openings? That is, a certain list of openings is picked (it could be publically announced prior to the tournament, or perhaps a surprise reveal), and then both players play games with those openings (each opening is played twice, once as black, once as white).

This seems like quite a fun thing to do and an obvious extension to the many forms of chess we already have? 

What's the drawback?
There are thematic tournaments.  For example, I recall a GM tournament where they had to play an open Sicilian.