An easy answer is that you physically can't play the game, the barrier of entry to watching a game on youtube vs buying a system capable of running it, the game itself, and a million dlc is like night and day. (Especially to teenage me, and now i just have the habit of watching them)

But that same logic of something being boring to watch is why i don't watch sports, but millions of others do and find entertainment in it. (And honestly its the same, watching someone else play a game that either you can't play, or at a skill level that you cannot achieve because you don't get paid to play that game)