Lots of reasons:
Less frequent bin collections
Takeaways - people dropping chips etc
People feeding birds - including putting out meat for red kite, which is madness
They are very intelligent, adaptable and they breed prolifically
They are developing immunity to the type of poison used
We have chickens and our neighbours blamed the rats on us - not aggressively but in a 'well if you have chickens..' However, she then admitted to putting grated cheese on the floor for the 'poor robins.' She also claimed to have a rat proof compost bin that was full of food - only to find that they had tunnelled in and had babies.
I am a conservationist and will not use a type of poison - which can in fact kill other creatures who eat dead rat bodies. But we had such a problem that I had to get in a professional - but one who understood the conservation problem.
I have very little issue with mice and actually, as soon as they disappear I worry - it means that the rats have taken over, as they will eat mice.
I am - and always have been - very particular. But when rats breed, the tiny babies can get into my aviary. I remove food at night and tried putting chilli powder into the food, which does help and does not affect the chickens.
I think to be responsible about waste food is important - but landfill is a breeding ground for them.
It's a shame really - they are so very intelligent. When I caught some (to release in woods very far from people) they at first were aggressive but when I came back (I gave them water) they responded to my voice and actually took food from my fingers. They are just a species trying to survive - so I don't understand why people hate them. That said, they are never ideal in a home and once they overstep that line - they have to be dealt with quite hard....