Because player dictates pacing in games game stories are broken in small pieces and tied to places or characters . In best case scenario sex in games is connected with a place to designate peaceful or friendly location (The Witcher) or as a ridiculous mini game, silly feature... (duke nukem , God of War, GTA, even Mass Effect as little bigger dialog puzzle). So it is always a place. For me that's missed opportunity to connect with the character, in some other way, and deepen the relationship. Sex in games done right simply starts more questions then in movies. Where is taken for granted just like the world the same movies create.
Let's take one simple example. Half Life 2. Alyx and Freeman. If Half Life 2 was a movie they would probably do it when they enter Black Mesa East. Just after meeting with her father. She is so happy she drags freeman to small room and grabs the wrench. Thanks to her engineering skills she takes the hev suit off. Freeman (starred by edward norton ) then takes her standing in typical "suspended congress" kamasutra position holding Alyx ( starred by Zoe Saldana ) ass with black gloves. (For some reason gloves and socks stay on at all times).
It would make sense in movie, but in the game? Stupid and meaningless? Or more importantly Jessica Alba as Alyx? Are you more interested in relationship between those two characters or Alyx ass? Are we human? Or are we wankers?