All videogames are software / programs, people say it is a video game and it is sold as a video game.No, not really.
It's a software / program. It doesn't really matter in which section steam puts it.
I once used a terrain generator called Terragen. Is that a game as well?
As a game, Space Engine lacks as many sim aspects as Elite, depending on your point of view. And it doesn't really have anything that would make it a game.
Space Engine is like your personal planetarium.
Also, you don't need to have all of the sim aspects of ED to be a sim.