Or you're designing a FS MTB and the suspension linkage wants to be in the same place a front mech would be. Or if you're SRAM and can't design a decent front mech. Or if you live in the flatlands so don't need the extra range a second chainring provides.
1x is stupid unless you have particular requirement for mud-shedding like in CX or you're looking for aero gains and running a large front ring.