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.

Keep in mind that the skills needed for game development were pretty much dark arts at the time. There was little to no formal education for them and few tools that weren't made in-house. So not only was the pool of people they could put to work immediately pretty small, the competitive market meant that companies were incentivised to produce the most impressive-looking games, meaning they didn't need merely competent people, but the freaks who could pull the most out of the limited hardware.

So they were worth their weight in gold, but companies didn't actually want to pay them that much.