If people want longer games, there are ways to force that. Play a safe champion, play for CS in lane (not kills), and help your allies when they are struggling (even if it's just roaming with 0 chance of a gank, to let them break a frozen wave).
When you have the option of losing a tower, or losing a tower + your life + the dragon spawning soon, just leave the tower to die.
It's not hard to play safe and extend the game. It's just that most people don't have as much fun playing that way. People just want to win lane and win the game, or "go next" asap.
I'm being a little unfair and exaggerating a bit. But you have to admit, some people play recklessly all the time, and that only helps make games shorter.