Why do many theorists doubt that genres exist at all?

Because the definitions are fluid at best, meaningless at worst.  

A genre which is derived from Latin “genus” is a style or kind of music, writing, motion pictures, or other media. A few genres are utilized in many types of workmanship or correspondence. For instance, the genres adventure and sci-fi are utilized in writing, motion pictures, and TV.

all cities have a governor

all cities are in states or provnies then country

all cities have people

difference

some are rich

some are poor

some are average

no questions on whether they are allowed that right