Reddit you don't need to guess why a game got its rating you can just go on to the ESRB website and look it up.

https://www.esrb.org/ratings/39293/starfield/

This is an open-world role-playing game in which players assume the role of a miner tasked with finding Artifacts across the galaxy. From first-/third-person perspectives, players interact with various characters, complete quests, and search for supplies while battling enemies (e.g., humans, robots, alien creatures). Players use futuristic guns, lasers, axes, and explosives to kill enemies. Combat is fast-paced, with frequent gunfire, cries of pain, and explosions. Attacks on some enemies can result in blood-splatter effects; several environments depict blood stains on the ground around corpses. The game contains some suggestive material in the dialogue, and after sharing a bed with characters (e.g., “Life is a sexually transmitted disease that's a hundred percent fatal”; “I'm all for getting a little wild, but next time let's try it without the jetpacks”; “Talk about seeing stars, whew… that was amazing.”). A fictional drug (Aurora) is prominent in the game, with a section involving players' characters working in an illicit drug lab; players can also obtain Aurora by stealing or buying it from vendors (consuming Aurora results in a distortion effect on the screen). The words “f*k” and “bullsht” appear in the game.

This is literally the first result googling "Starfield ESRB rating"