Why is Mangia "he eats" and Lei Mangia is "she eats." Why isn't it Lui Mangia?
Mangia could be He eats, She eats or It eats. Lui Mangia can only be He eats. Lei Mangia can only be She eats.
You should be able to determine which one by context.
Just to add to this. If you really want to specify 'it' eats (there's almost never need for this), but if you did want to say that you could say
- esso mangia
- essa mangia
depending on the gender of the it you're referring to. =D
I agree with BelgianStr.
I also want to note that in most languages, if you are not sure of the gender of the subject because only the verb is being used, speakers generally default to the masculine. In Italian that would be lui.
It's a context thing. So if you were talking to someone and the conversation was about a boy, you could say Mangia and the other person would understand what you mean. Therefore, the sentence isn't incorrect as much as it might seem kind of odd without context behind it. It's not like in English where you can't say "eats" and have it be grammatically correct, because you need the "he", "she", "they" part of it to make it a correct sentence. In Italian you don't need that so there are certain sentences that don't make much sense unless you know the context.