This comes from you, it's not a rule and it's not common.
Gens are usually defined by similar tech, sometimes are bits, sometimes are polygons, sometimes are number of cores, etc.
NES and Gameboy are associated with the same gen not (only) because the era they lived in, but also because the architecture was very similar in the way it worked (CPU+PPU).
PCEngine is "often" (not always) associated with the 16bit era because of the colors, scrolling, parallax, sprites, etc.
XBOX and PS2 is a strange case, despite their totally different philosophy, polygonal bruteforce vs shaders, exotic vs PCish hardware, they where both a totally different paradigma from the 32bit era, so they belong to the same gen.
Some people consider Jaguar and 3DO a "pre32bit era", but 3DO had lot in common with Saturn and PSX.
Switch can't be considered in any form a PS360 era when there is a 10years gap in tech and graphics approaches between them.