Because the simple present of french can be translated into a simple present o present continuous to english. And it's perfectly acceptable and it's how they communicate.
If somebody asks you what you're doing and you reply - "Je mange du pain" they know that you mean you are in the process of eating it.
There are other tenses in french that express a continuous like the "être en train de" structure, but they prefer to use the simple present because it's shorter.
Or the present participle = "en -ant" form, but this one is used sparingly and has specific rules to be used.
Thank you for your response but I am not sure if it answers my question. Normally for these type of questions Duolingo accepts ' it eats or it is eating'. If that was the reason my answer wasn't accepted then it would be very inconsistent of duoling to do that.
My problem is with the usage of 'il'. This can mean it or he and without context how do we know which it is. I wrote 'he' and it wasn't accepted and I don't really understand why.
In those cases you don't use the discuss section, you use the report function.
Although, my guess is that, since this section is about animals, this sentence refers to another animal, maybe "un requin" that is eating the dolphin. That's the only context we have here anyway, "animals section". But, again, you should try reporting it.