Overconfidence is the Empire's weakness
It seems that the Empire was counting on people being too afraid to even try anything. Even after the Rebels stole the Death Star plans, the Empire was doubtful there would even be any weaknesses to find. They seemed confident that the Death Star's superlaser was sufficient.
MOTTI: Dangerous to your starfleet, Commander, not to this battle station. [...]
TARKIN: The regional governors now have direct control over their territories. Fear will keep the local systems in line. Fear of this battle station.
TAGGE: And what of the Rebellion? If the Rebels have obtained a complete technical reading of this station, it is possible, however unlikely, they might find a weakness and exploit it.
VADER: The plans you refer to will soon be back in our hands.
MOTTI: Any attack made by the Rebels against this station would be a useless gesture, no matter what technical data they have obtained. This station is now the ultimate power in the universe. I suggest we use it. [...]
TARKIN: This bickering is pointless. Lord Vader will provide us with the location of the Rebel fortress by the time this station is operational. We will then crush the Rebellion with one swift stroke.
General Dodonna, the Rebel leader giving the mission briefing, says the Empire doesn't consider the Rebel attack to be a threat.
DODONNA: The Empire doesn't consider a small one-man fighter to be any threat, or they'd have a tighter defense
This is backed up by the fact that initially they didn't even send out any fighters to take out the Rebel ships, figuring the turbo lasers were sufficient.
ASTRO-OFFICER : We count thirty Rebel ships, Lord Vader. But they're so small they're evading our turbo-lasers!
VADER : We'll have to destroy them ship to ship. Get the crews to their fighters.
Even after being told that there indeed is a danger, Tarkin is confident the Rebels don't actually pose any threat.
TARKIN : Evacuate? In our moment of triumph? I think you overestimate their chances!
It seems Luke was right when he confronted the Emperor in Return of the Jedi
LUKE: Your overconfidence is your weakness