Your experience is interesting because I played Diablo 3 when it first came out and shortly afterwards got bored because the loot wasn't interesting. Once I finished all of the difficulties in hardcore mode (before everything got nerfed beyond its initial release) I uninstalled. But, I somehow managed to put thousands of hours into Diablo 2 and Path of Exile, both of which have much more interesting items (at least to me). Maybe the reward aspect is more important to me in those types of games.
I get what you mean by the gaming tasks though. I used to play moba style games. Every second had something to think about, and really it's almost like you have to turn off your brain and just let your brain take over if that makes any sense. If you tried to explain the exact work flow of what you had to think about in 5 seconds it almost seems impossible, but somehow with enough practice it's effortless. Maybe when your brain is in that state, it's not possible to get distracted.
The idea of loot and reward is a good theory but I meant it only in the sense of gameplay, and I can prove it by "loot lockout": in World of Warcraft you can only get loot from a raid boss in a specific difficulty once a week. The next successes give you no reward (and you can't trade loot with someone who did get loot when you couldn't). However, I've still cleared the same bosses several times a week because they were also fun. It was guaranteed that I wouldn't get immediate -- or any -- rewards for my success but it was still fun and focusable.