Just because Seven can do the Job, she's still not a Starfleet Officer. They make sure that you're decent, honourable,tolerant and not psychotically crazy. It's a serious process to make sure that they continue to think the right way even in the worst situations... Although, sometimes a few headcases slip through because their parents are top shelf.
There's a %60 chance that Seven of Nine is a Borg Sleeper.
Meanwhile there's a %30 chance that she's been the Borg Queen the whole time laughing at the crew of Voyager from a quiet place in her soul.
It's bad enough that Tom was First Officer in Resolutions.