Middle aged man cheating on young girlfriend with middle aged woman

Ask his girlfriend.

:-) but
seriously, why? i'm just very curious...

Because he likes variety and you don't mind spreading them for him. That is my guess.

The question is WHY is he still with his girlfriend?
Is she for show and tell but bore in bed and you are
good in bed but he wants to show young goods?
If it is - pretty shitty man he is.
If he likes you very much - WHY is he still with his girlfriend?
Do they have kids?

And NO this is not a 'common' thing. I don't think so.
Opposite is common thing, not your story.

didn't you answer your own question?

What's a 54 year old man going to talk about with a girl 21 years his age?

It's the pillow talk - you do it better.

(He may be having great sex with her, too. But you have more in common with him. )