Like I said, our understanding of what species are is lacking. How do we really know tigers and lions weren't bred from an ancestor cat by an ancient civilization? Did you know that our entire living organism classification system is based on assumptions of experts, and came about before the discovery of DNA?
The reasoning they give for tigers and lions being able to mate is because they are in the same genus. But they were grouped into the genus arbitrarily. There is nothing in DNA that says what kingdom, phylum, class, order, family, genus or species a creature belongs to, because all those things are made up. But our understanding, and continued research, of the relationship between animals is based on those centuries old assumptions. Biologists know this, but still cling to dogma. What can you do?