Even a stopped clock is right twice a day. (not referring to Wyden)
Really, the best reason (from the security org point of view) not to collect so much surveillance on your home population, is because it creates a single point of failure for foreign adversaries to gain access. Most of the interesting data is already commercially available for advertising.
Because to keep their jobs they have to be seen as doing something and the old 'this is something therefore it must be done.' truism still hold. They never look farther ahead than the next election, were lucky if the look farther than the next nights news most of the time
Really, the best reason (from the security org point of view) not to collect so much surveillance on your home population, is because it creates a single point of failure for foreign adversaries to gain access. Most of the interesting data is already commercially available for advertising.