Unfortunately, they have to run these things and be so strict because some people are really, really stupid.
I know a case where a guy brought his wife into his office and got fired for it. That's what he claims, anyway.
The longer, accurate version? A contractor with a high security clearance brought his new bride to his office, bypassing security. Strike one. When they got there, he left her in his office while he went off to do something. Strike two. His wife looked at his computer- his UNLOCKED computer (strike three)- and saw his desktop was a nude pic of another woman and the email message that popped up while she was standing there was decidedly raunchy. When he came back in, they got into a fight, culminating her beaning him with the keyboard. He left her there again and went to the health unit. Someone from security spots the blood trail, chases down the pair of them and brings them to the guards desk. Wife is still carrying keyboard. She tries to clock him a second time. Someone runs his information while they are dealing with this and calls his company to report the problem. They respond, "Wife, what wife? He never told us he got married!" Strike four.
When you have a clearance, change in family circumstances has to be reported immediately. You get married and don't tell- you've just created a pressure point for someone to exploit.
He was removed from the agency, fired and lost his clearance. Try getting another job in your field after that.
He's also now divorced.