This is a common question and the simple answer is... the previous guys didn't effectively complete the job.
Pest birds are relentless, just like you would be if someone was trying to push you away from your food source or home, this means we have to be relentless too. We build an action plan based on the pest and the problem at hand using every trick we have, these plans tend to include random visits at random times, with the correct birds for the job. Along with nest removal, night visits and so much more.