Games were small back then, so there was less often an ongoing relationship between a publisher and a developer, and more often just a one-time exchange: software, already finished, for a lump sum. As if the publisher had contracted out work on the game and didn't know it until the contractor showed up at their door. Basically, games as "piece work."

At that point, the developer walked off with money, and the publisher walked off with a game binary they could resell and market however they liked (including taking the IP rights and licensing them to a porter.) But the publisher didn't retain any right to the developer's time; and often, the "developer" was just some college kid who developed just the one game, moved on to other pursuits, and now can't be found.