Why are crankset "sizes" often defined by the amount of cassette sprockets? For example: https://bike.shimano.com/en-EU/components/road/category/crankset.html

Almost all of the listed cranksets have sizes listed as 2x11/3x10/3x9 - why does the amount of cassette sprockets matter when buying a crankset?

Is it physically impossible to pair, for example, a 3x10 crankset with an 11-speed cassette?
The distance between chainrings (and possibly design of ramps and pins) is determined by chain external width, which is determined by number of speeds in the rear. Too narrow (higher number of speeds) and you will have chain rub, too wide (smaller number of speeds) and you will risk jamming the chain between chainrings. 

I have been riding 2x10 crankset with 9 speed chain for years without problems, but on the other hand I have also seen the chain jamming which was took quite some time to fix on the road.