Tons of Steam games don’t have DRM, so you can just run them directly without Steam running. Many platforms enforce their DRM in all their games, so you need their app running to be able to launch games. Steam doesn’t enforce anything like that, their DRM is completely opt-in.

I don’t know if older Ubisoft games are available from Uplay without the DRM, but I would be very surprised if anything released in the last 10 years or so doesn’t rely on their launcher.