Can someone explain to me why, over time, democratic states tend to drift into mass surveillance ?

Is it beacuse a lot of people feel unsafe or is it because the people supposed to ensure our security see it as the "easiest" or most effective way to do their job?

Is there so much benefit to having a fuctioning mass surveilance apparatus, and if yes, who benefits of it if not the people for whom these rules for in the first place?