Not the person you are replying to but...
In my case was because if I tried to give space to older people, I would go nuts, having ADHD and possibly (I never tried to get diagnosed since there is no treatment) Asperger's, I just don't notice old people there... So I go, see a spot, sit, and after a while note the stares and so on.
I had to learn to ignore the stares, because whenever I was worried about being a gentleman and whatnot I ended going into crazy thinking spirals on my head, ruminating non-stop if I was offending someone or not, or not noticing someone, I would instead scare people, looking like some sort of paranoid person on the bus.
The alternative coping mechanism would be not to sit at all if most of the sitting spaces are already taken. I sometimes did so as I expected to have to vacate my spot in only a few minutes.
ADHD not, but CFS yes - i take meds daily to not keep falling asleep all the time.
My theory is that for the part of the ADHD population the core reason is excessive daytime sleepiness/tiredness.
Why? Giving up a seat to an older individual is a show of respect as well as a recognition that standing for seniors is a bigger burden than it is for a young 30-something. Why would ADHD change that?