Parents evenings are usually organised by timed appointments, so parents who can't get there earlier take the later times. These events start at 4 or 5 pm because the teachers are on the spot at that time, and if they went home it is quite likely that they couldn't get back by or 7 or 8 pm, or fit in the interviews with all the parents during the evening. They have already been speaking to children for eight hours or so, and believe me, after several hours more speaking to parents with no more sustenance than a grabbed sandwich, a late night is the last thing they need.
"more time to plan and mark, without the distraction of having to teach" You could put that the other way round - more time to teach without the distraction of planning and marking - as Gilly says "isn't "teaching" what "teachers" are paid to do?". Neither of them can be done at the same time time as the other, so the planning and marking are done at home or in the holidays, in all that "spare time" that teachers don't, after all, have.
But, to gilly again " a class full of children all taking "their week" at a different time. Imagine trying to keep up to date with the curriculum?" is exactly why parents are discouraged from taking their children away during the term.