Historically, developing games for three distinct operating systems was a logistical nightmare for developers. Windows uses DirectX; Linux and macOS rely heavily on OpenGL or Vulkan. The file systems are different, the hardware drivers vary wildly, and the user interfaces require distinct optimizations.
Looking for a specific recommendation? Check out the "Cross-Platform" tag on Steam or the "All OS" filter on Itch.io. Your next favorite game is just a download away. A Downloadable Game For Windows Macos Linux
~14 weeks (3.5 months) for a small (2-3 person) team. the hardware drivers vary wildly
If you are looking to build a game that targets all three platforms from a single codebase, these tools are the industry standards: Godot Engine A Downloadable Game For Windows Macos Linux