Two reasons: 1) Because storing spent nuclear fuel on earth isn't actually hard, it is just brought up anytime people talk about nuclear because it's the obvio

Two reasons: 1) Because storing spent nuclear fuel on earth isn't actually hard, it is just brought up anytime people talk about nuclear because it's the obvious thing to talk about. 2) Rocket fleets have a ~97% success rate (e.g. see Vega C this week). Spreading 3% of the radioactive material globally isn't a great idea.