[grotorea]

Some of my guesses:

Programmers are overrepresented both among FOSS enthusiasts and among nostalgic old gamers, meaning there are more people able to and interested in rewriting the engine code collaboratively than there are for the other parts of game-making.

People who want to do unpaid work on the other parts of game-making can do mods instead.

Making a new game is an artistic endeavour that is hard to do without either having an hierarchy making the decisions or being a solo/small group thing, making it somewhat unfit for community FOSS. When doing a remake you have the specification more or less done, you just have to implement it and do the uncontroversial fixes and improvements.