Tarkin didn't want to "evacuate" because they were only moments away from destroying the rebel base, with only a few scud fighters trying ALMOST hopelessly to exploit the faw of a straight exhaust port directly to the core. If it weren't for Luke's heroism, they would have triumphed. It wasn't a foolish move. He thought it was an exceptable risk to stay. I mean, the rebels chances of winning that battle were about, I don't know....3,720 to 1...
Not to mention if Han hadn't have come back, Luke would have been shot down.The "danger" that guy found when annalyzing their attack plan was VERY remote, and Tarkin knew it wasn't worth evacuating the whole base and abandoning the plan to crush the rebellion on the very slim chance that the rebels were successful.
Basically,
Imperial dude: "But sir, it's possible that their last remaining fighter will have a force adept pilot with Obi-Won Kenobi as his guide, and Vader won't be able to blow him out of the sky because Han Solo is going to come out of nowhere and make his wingman panic and accidently clip his fighter, sending him hurdelling into space."
Tarkin: "Uh, are you retarded?"