Some maps dont have custom spawn points, because they are older map mods. The spawn system with multiple spawnpoints came just a few updates ago. If you are using a map that is old enough, it will not support this feature
I actually mean that it disables the menu to select where you want to spawn, if the map doesn't support it.
... yes, it is. In fact I still find the entire process of starting the game, getting into a map, and selecting my car to be quite a drag overall. I doubt that the devs are finished working on this feature though - it currently doesn't behave exactly as described in the initial release notes. I expect we'll see what @Justy4WDTURBO suggested at some point.
I dont know why. IMO BeamNG takes little to nothing to start up and run compared to several games, specially GTA V.