Cops don't tend to be involved in fly tipping which might explain why they're not bothered by it. It's normally enviromental health (I think it's called) who deal with it, and may call on the police for assistance but the police themselves don't deal with it.
Given that we've had 12 years of services being cut to the marrow it's not really surprising that they're not interested in relatively smaller amounts, or where it's probably not a danger to the public/blocking public rights of way, they simply don't have the staff to deal with most of it, so they tend to go for where there is repeated dumping/signs it's being done by someone doing it commercially.