In French if I write the sentence
Je mange le déjeuner
it would/could be the same as if I am saying I am eating lunch. What is going on in French goes on in a number of the other Romance languages.
How is it that it came to be in the English language that "I eat lunch" cannot represent something I am doing right now?
Steven's second paragraph, about blocking, is not quite correct. Spanish for example has both a simple present (él come) and a present progressive formed from a copula + gerund (él está comiendo), however the former is the default expression of a punctual action (at least in conservative dialects). I have heard it claimed that in some innovative dialects the gerund is in the process of replacing the simple present.
As a matter of history, Old English lacked a copula + gerund form, and developed it over the course of its development into Middle and Early Modern English. This development was not uniform. One of the last constructions to gain a morphologically regular progressive form was the passive. In earlier English, the sentence in (1) was ungrammatical, with (2) being the preferred way of expressing the meaning:
That example was taken from this Language Log post, which also has an illustration that people were objecting to this innovation as late as the latter half of the 19th century (the use of the progressive in other contexts was well-established by this time).
Perhaps you might get a more pointed response in the English Language and Usage StackExchange.
As for the question itself, my first instinct would be that the potential continuous meaning of the simple present I eat lunch is blocked by the existence of an unambiguously progressive form I am eating lunch. We see this pattern all the time in semantic/morphological blocking.
Other Germanic languages, to my very limited knowledge, do not have a similar progressive verb form, so I would reckon that it is a new development in English. Moreover, the auxiliary + deadjectival verb nature of the progressive strongly points to it being a derivative form. It's probable that somewhere along the line, people decided that is X-ing was a good way to specifically refer to a continuous action, and the emergence of the new construction pushed out one of the original meanings of the simple present.
My proposal is not necessarily true, though. It could easily be the case that the progressive forms emerged in response to the lack of the continuous meaning. That's a question an English historian might be better able to answer than me.
