Or a cop happens to be sitting one street over and hears a gunshot and gets to the scene just as you're stepping out of the building.

Or the victim is paranoid and has an itchy trigger finger.

Or the cops are investigating the victim's family and associates and notice a weird pattern in their bank account.

There's a lot of ways a killing can go wrong. Even if the odds of any one killing going bad are small, the cumulative chance of an error creeps up with each job. Worse, it's hard to jack up your prices really high to reduce the number of jobs you have to do because the more money moves around the more scrutiny it receives.