For example I was looking recently at some Petrosian games and realized they weren't in the lichess opening explorer for masters. Out of curiosity, is there any reason for this?
Somebody has to import the games. You can import them.
Huh? Anyone can just import master games? That doesn't make sense. How does lichess determine whether an imported game is a master game or just a user game then?
It may be more useful to use an external program like SCID vs PC http://scidvspc.sourceforge.net/ and game databases like www.kingbase-chess.net/ for millions of games or http://www.pgnmentor.com/files.html for games from specific players. All of this is free.
Will make it much more useful if you're trying to emulate or research openings/positions/endgames of current/recent play or specific player(s). Especially helpful to see what GMs do in positions you may run into in your games.
TheRatRiverTrapper has a point. ChessTempo has a curated database and an opening explorer - there is no reason this could not be done here.
You can't post in the forums yet. Play some games!