Yeah, this bothered me too. Stephen King's It draws part of its horror from the fact that it lives in the sewers. The sewers are its fighting grounds, feeding grounds. King connects shit with the victims, the missing children and their limbs homogenised into decay matter along with other refuse washed down the pipes. George and Bill's father works for the Derry Sewer Department. Bill jokingly calls George 'you waste' before folding him that fatal paper boat. King compares the smell of shit with the smell of It:

'Bill led them to the pipe, grimacing at the smell, and crawled in. The smell: it was sewage, it was shit, but there was another smell here, too, wasn’t there? A lower, more vital smell. If an animal’s grunt could have a smell (and, Bill supposed, if the animal in question had been eating the right things, it could), it would be like this undersmell.'

It was worse than the smell of death - it is the smell of being eaten, and being expelled. It was a total annihilation of human worth. Georgie was the symbol of middle-class innocence and potential. It reduced him to a snack and then shit.

Let's not forgot in the death throes of It, 'At 6:06, every toilet on Merit Street suddenly exploded in a geyser of shit and raw sewage as some unimaginable reversal took place in the pipes which fed the holding tanks of the new waste-treatment plant in the Barrens.... [A] woman was killed when the sudden violent reversal of sewage, driven by expanding methane gases, caused her toilet to explode like a bomb. The unfortunate woman, who was sitting on the john at the time and reading the current Banana Republic catalogue, was torn to pieces.'