Except "stuff" isn't coming out of the back of aircraft: they're talking about aircraft contrails which is just condensed water vapor from wingtip turbulence.

The people who claim they're monitoring chemtrails aren't even watching aircraft which are deliberately dispensing payloads, because it just isn't that common in the first place (unless you go out and watch crop dusting, but then you can also just see the guy land, get out, and talk about it).