[mshook]

Because you need to vent the pipes. Otherwise the sewer system would just be a giant vacuum pump.

It's even described in the article you mention.

https://en.m.wikipedia.org/wiki/Drain-waste-vent_system

[flaviu2]

I'm familiar with vents, but indoor vents are basically one-way air valves. I don't know if this is required by code, but it's definitely common practice (you don't want poop smell coming out of your vents)

[Scoundreller]

Vent stacks need to be above that roof’s line, but your building might be next door and taller with open windows downwind.

Or a crack in the stack.

[BubRoss]

That isn't a good explanation, since venting should exit far above the building.