Variety of reasons I believe: anti-poaching, preferring to go by their gamer tag, not really industry standard at the time (Yoko Shimomura for example did a lot of iconic early Capcom stuff but it was credited as “Capcom Music Team”), gaming seen as a “lesser” industry so people didn’t want to be associated with it, corporate egos, anti-doxxing (yes, even back in the day. Gaming was a small industry). It really depends on the country/company etc.

It’s actually gotten to a point where certain older games getting ported are actually entirely credited to the porting team rather than the OG creators. I know the Metroid ports had their OG creator credits removed (not sure if they were patched back in or not), and it was a minor kerfuffle. It’s a topic discussed in game preservation spaces with some frequency.