Technically, it is: https://en.wiktionary.org/wiki/sentient (the first two definitions). You mean the science fiction definition though (the third), and it's because it's not self-aware and not very intelligent, because it doesn't need to be to survive and procreate successfully. A slightly smarter crocodile must not have enough of an advantage over its peers to matter for evolutionary purposes.

Who says they aren't? Of all the reptiles, they seem among the smartest.