Because matt paint seems to be the "in" thing just now just like white paint was a couple of years ago.
There is a difference between Flat and Matte though!
With Matte you'd get this 'open' paint surface... with Flat the paint surface is 'closed', like Gloss. See it as a Gloss with a Flattener added!
Less build up of dirt. (when new use bumper snot / bumper care for a barrier.)
With a Flat Clear Lacquer you'd still use the same Hardener as with a Gloss Clear Lacquer.
It's a bit of a trade secret mixing and applying Flat Clear, wrongly applied and you end up with a Semi Gloss.
IF you're a carpainter you know what i'm talking about.
ps. in the door card inserts the 'door closers' are topcoated with a Flat Clear btw. And many household appliances are covered with a 'soft touch' flat clear, ding resistant and tactile.