Most likely because hybrids are extremely uncommon in the wild, the only ligers or any hybrids have ever been in forced captivity, whereas a lion and a tiger would never meet in the wild - they live in different continents! And I don't need to even explain the Quagga and the zebra :D If we made hybrids as common as normal babies in the game, you might as well just chuck reality out the window :D
Although maybe a 1:1000 chance, it would be pretty nifty. But you couldn't breed them again :D Maybe like the killer penguin...