Inter-specific crosses are rare in nature and inter-generic crosses almost unknown .why?

Explanation

in interspecific crocess , male and female animals of two different relatd species are mated , in some cases , the progeny may combine desirable feateures of both the parents . And may be cosidereable economic value e.g., the mule .
An interspecfic hybird 's hybird fertily varies Many of panthera (,ligers , togous lepons etc. ) hybrid males are infertile , while the female conuterparts can breed with the species she contains (female ligers can breed with lion or a tiger to produce li- ligers or ti- tigers ) . these are rare in nature .
interheneric hybridisation is the crossing of two different animals / plants of different genus . it is almost unknown in nature ,intergeneric hybrids more or less never form complete zygotes/ embryos . these are sterile / inferrile .