The Phasers actually DID have a gunsight.
They just rarely ever used it (probably because only the hero props had them). You can see it in action in The Man Trap when Spock turns the dial on the phaser 1 atop his pistol and the sight pops up.
The phaser 1 is an ergonomic nightmare, however. The trigger is on the bottom, and to hold it in your hand and be able to push the trigger makes it nearly impossible not to block the sight with your thumb. Jefferies designed a cool looking little prop but it was impractical to handle.