The modern rules of chess, except for the stalemating being a draw, are basically equivalent to a game in which the goal is just to capture the king, not checkmate him. It's plausible that in the oldest versions of chess, this was indeed the goal, but since a rational player would then always move their king when in check, and never leave themselves in check, and would only have their king captured when there was no escaping from it, players may have decided that responding to check may as well just be made a rule.
I haven't been able to find any reliable source either confirming or refuting this hypothesis. Is there any?