"Because many people are "known" to them. It simply isn't possible to monitor all of them, short of internment."
Loony neighbour reports are easy to deal with because there will only be one or two reports about the person of interest. You can eliminate that kind of noise pretty easily.
However, when someone is being reported by multiple sources - and those sources include credible people like various Imams (and the head of one of the UK's largest anti-terror muslim charities telling them "This guy is off his rocker and dangerous, you need to act NOW"), then filing it under non-urgent isn't appropriate.
That said, the security apparatchik is understaffed and underfunded, but not making best use of intelligence provided isn't helping their cause (FWIW, internment of potential terrorists would cost at least 100 times as much as the shortfall in funding now being talked about, probably 1000 times. There are sound financial and psychological reasons to avoid doing it)