Well it can be 10-20+ minutes just getting into a game. Then add in potentially 30-45+ minutes for a game that can either be very rewarding or deplorable and yeah, the time consumption to enjoyment factor really starts to go askew.

Let's think of the time it takes to actually have your queue pop, then have the new, convoluted champ select system go through the picks/bans - oh somebody dodged. Repeat. Oh somebody didn't lock in, repeat. Oh one of your friends (or you) have a 5-15 minute queue cooldown for missing a lock-in -- the list can go on and get very ugly.

Very rarely does my queue pop and my champ select go through perfectly. There's typically one or two dodges and after a second dodge, maybe you rush off to the restroom and miss locking in, etc. Long story short: even things that aren't game play eat up time in League.

Let's say you get into a game and it's a solid, hard fought game that feels so high in quality: people are pinging, calling out, warding, upgrading trinkets, not playing foolishly, not flaming. Awesome. Such a rewarding gaming experience. Maybe you get into a game with some guy on tilt who ruins the game for 9 people. Maybe you get into a dynamic queue 5v1 fiesta. Maybe somebody trolls champ select, AFKs, DCs honestly (like with internet issues and the such). Then all that time was literally wasted and you start to tilt yourself.

Lots and lots of factors can make League unpleasant and the time invested feels awful. In Overwatch, you have minimal time in game (comparatively speaking) and getting into game. You have less salty people as a result and more people are just going with the flow. You lose? Oh well, get into another game quickly and try again; have fun. In League after a really terrible game with a rager or something you just want to punt your computer into oblivion and uninstall. I never had this with Overwatch, it was just pure fun and it's not the beta aspect: I've played in some miserable betas.