AITAH for not wanting to celebrate my step kids' spouse's birthdays?

My step kids are in their 20's.

They have boyfriends/girlfriends.

My stepdaughter's boyfriend: his parents don't celebrate his birthday anymore (he's 23).

My stepson's girlfriend: same thing, she's 22 and her family doesn't go all out to celebrate her birthday.

They've only been dating a few months, not even a whole year yet.

Why am I going to waste 1 of my days off to decorate, cook, get a gift, etc. for them just cause their parents are shit?

Does that make me an asshole?