Why is moving poop through a city harder than just sloping a pipe downhill?

Why is moving poop through a city harder than just sloping a pipe downhill?

In “How Sewers Work (feat. Fake Poop),” Practical Engineering explains why sanitary sewers are high-stakes public-health infrastructure: they must estimate future flows, survive storms, use gravity without clogging, navigate difficult terrain, and cope with things people should not flush.