Because either those developers are long gone, or in their eyes it's not worth it due to how little people play on macOS versus Linux and Windows, or they literally can't due to a technical issue. Valve's old Source games for example are all 32 bit due to how old the Source Engine is. They tried to update Half Life 2 to 64 bit, but it did not work right at all. It crashed constantly and had horrible framerate that the 64 bit update was scrapped entirely. It's why CSGO ran so poor on macOS nowadays since Catalina forced them to haphazardly update the game to 64 bit, on an engine never meant to run 64 bit binaries to begin with
Fortunately there's a solution for Mac users who want their library of games: The Valve Steam Deck, which just got refreshed to have 1TB configurations and a new OLED model with longer battery life. Combined with a price drop for higher configs it's never been a better time to hop on the handheld Linux PC that could.
Available now.
www.steamdeck.com
Or just check Apple Gaming Wiki and see if your game works through GPTK.