I am one of those with a season ticket who pick and choose what games I attend. It's a three hour drive to the Etihad and time away from my business; add to that my son now lives even further and is often abroad. For certain games we will ensure we have no other obligations and a lot of other fixtures I/we can make anyway. I used to often pay top dollar to friends/touts for big game tickets, so the cost for 14/15 games a season seems ok to me.
My first season ticket was in the Platt Lane in '75, so I've done the time. I used to sell on Viagogo, but now it's a couple of quid off next year's ticket I don't bother (might not sell anyway) - especially as the match day 'experience' is making me consider not renewing (but that's another thread)
I'm sure a lot of the empty seats are due to similar reasons. I think it's something we've got to accept and not get wound up by the school playground 'Emptyhad' jibes.