Why do flies only land on things that smell

Some flies are decomposers as maggots. So bad smelling decaying organic matter it what they eat. But you also have other flies that suck blood like horse flies and mosquitoes and no see ums, and then you have nectar feeding flies known as Syrphids. But within all that you have flies that are predatory hanging flies are predatory and robber flies. So not all flies land on bad smelling things.

Can't be answered since the premise is false.

Flies land on all kinds of things: Check out a dozen or a hundred of these and see how many landed on something "smelly:"

http://www.pbase.com/inaturalist/insects…

True, Scathophagids, Sarcophagids and sometimes Muscids land on smelly things, but most species do not.

There are plants that smell like rotting flesh, because this is their way to attract flies to help pollinate them. The odor works because some flies are scavengers, and they are genetically programmed to be attracted to the smell of rotting flesh.