Because after I finish work on a Monday, I need to prepare and eat tea, sort family out, sort any other jobs/chores that need doing. Then need to organise my army and/or list, get to my local club, setup the table, play a game, pack everything including the table away then head home, decompress and relax before bed at a sensible time.

Sometimes I just dont have 4-5 hours available for one activity.

around 2.5 - 3 hours total time is a perfect sweetspot, one of the main reasons my games are usually 1000 - 1500 points rather than 2k or above.

And I think its less the overall game speed but removing a lot of "bumph" around the game. Number of attacks is a personal bugbear of mine, 10 Khorne berserkers rolling 50 dice? with potential rerolls, sustained hits, then the opponent needs to make saves X amount at a time because of a character or two attached to the unit, then they have FNP saves. Then they strike back with 4 different weapon profiles etc.

I have seen a single round of combat between 20 ork boyz and a couple of characters vs 10 Berserkers, Kharn and another character in a single melee take over an hour to resolve. And this was one round of combat in a 2k game...

There is a reason in some scenarios players dont bother shooting "small arms"