It's generally too cramped to do that politely, especially if you don't have an aisle seat, they're in the middle of food and drinks, or there is a line for the bathroom. If the plane is large enough to have areas where you can stand without getting in someone else's space, the flight attendants don't let you mill about for long.

Yes, they were not yet tired enough of sitting, at least not enough to stand up for some small amount of time, only to have to sit back down again. This is true for any part of the flight, not not just before the start of descent.

There are also any number of other reasons those individuals may not have been motivated enough to stand during the flight itself, as a sibling comment pointed out.

My point was that the original observation was not actually of anything odd but, instead, of something consistent with this particular motivation.