Call them ‘Mother-in-law’ and ‘Father-in-law’. Full title, every time you address them. It’s a trifle long, but has the virtue of being accurate.
Or, depending on how smart-ass you’re feeling, the following:
“Y’know, I’m not comfortable calling you mom. I already have one of those. So how about I call you ‘uncle’ instead? Sure, you’re not my uncle, but you’re not my mom either. And an uncle is a family member, so I don’t see the difference.”
More seriously, if I may ask:
How old were you then? I was practically a child when I got married & probably should have called my in-laws “mom & dad” for all the guidance & help they gave me. I’ve always called them by their first names, and while they were a bit miffed about it at first, they’re OK with it now. Partly, I think, because I grew up Of course, YMMV.
Both my wife and I were just shy of 22 when I proposed and we were both in our 3rd year at college. I’m positive that they didn’t see me (or their daughter) as an adult. Maybe we’re still children in their minds?
This is your in-laws problem not yours. My now ex in-laws were upper middle class bordering on rich and kind of snooty but they insisted that I called them by their first names.
It sounds like they are unlikely to change so the only advice I can give is grin and bear it. As you described it, your wife doesn’t seem to want to bring the subject up with her parents, leave it alone man. Pick your battles, this one is a loser.
Hah! But people typically grow up with more than one “Grandma” and “Grandpa”, versus the people they call “Mom” or “Dad”. The latter terms tend to be reserved for singular and pretty special people, while with multiple “Grandma”/“Grandpa” possibilities, you start thinking it’s more of a general label you could start slapping on anyone.
Of course, YMMV.