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.