Because he's paranoid. The fact that he frisks Michael himself when he stands up, even though he was already frisked before his eyes shows his paranoia. This is the man whose father he tried to kill. As far as Sollozzo knew, Michael could have come back with a kitchen knife. He's not gonna take any chances.

Clemenza actually predicts that Sollozzo would try to frisk him after he comes back with the gun, and that's why he tells Michael not to sit down again but to immediately blast them and leave.

However, Michael does sit down and it's Sollozzo's carelessness that gets him killed. But the problem is, Sollozzo would never display such carelessness.