That's the main problem. Personally, I often work until 7pm, and I need at least an hour to ramp down and have dinner before we start. Similarly, as @Burnside mentions, the players with kids can only really start once those have been put to bed.
RE: a weeknight session I think it depends on a few factors. First whether its online or in person, peoples availability/scheduling, campaign type and DM preparedness, amongst other things. Our group meets in-person every other week on Mondays from 4PM until we reach a point that's ideal to quit and reconvene next session. On the average I'd say we get together for 3 to 3-1/2 hours sometimes less sometimes more, but we do get sidetracked quite frequently during the game, with someone cracking a joke, something in the game causes us to deviate the topic to something real world, etc. So, we usually get in 2 to 2-1/2 hours of actual gaming which is fine with us. we do take the game seriously (to an extent) but we also play just to get together, have some drinks and hang out. As long as the adventure is good, which it usually is, we're not overly concerned with how much we complete or accomplish. We run more of an episodic campaign than over-reaching arcs with an ultimate end game, so we don't have any expectations of moving the campaign forward we just see where things end up. It's fun and worth the few hours a couple times a month to play. I think the question you need to ask yourself is whether you're playing either 2 or 4 hours on a weeknight, is it fun and worth setting aside your time to play in the weeknight game rather than doing something else.
