It's really not necessary, especially with white. Before that move 17 you can deviate at virtually any point and end up with an even game. As black it is harder

It's really not necessary, especially with white. Before that move 17 you can deviate at virtually any point and end up with an even game. As black it is harder to be really equal without knowing theory against an opponent who does, but black has a huge information advantage (you only play 1 obscure defence to 1.e4, white has to know them all). What distorts your view is that many chess players _like_ to study theory. The Sicilian with its famous counter attacking situations! The King's Indian with its famous attacks on opposite flanks and a closed center! I have seen many GM games like that and I want to play like that too. When both players are like that, they play into really sharp and hard to understand lines, where it really matters if you know the theory one move deeper. There it does, because such positions are often extremely sharp, and as you _can_ prepare them often, that is worthwhile. But in the other games where you meet an opponent who plays something quiet that you've never seen before on move 3, it's all useless. And such moves are a perfectly legitimate and strong choice. Anyway, go to any chess club, and ask around which players have memorized the most opening theory. It'll be the 1700 guys, not the masters. Memorizing theory doesn't actually make you stronger.