the argument is that these exchanges were based on different social arrangements so that pre-capitalist societies did not have a value-logic at all, even if they had the buying and selling of goods. "worth" in these societies was not a purely economic notion in the way that it becomes in a world of generalized commodity exchange. there's a reason that people debated for centuries about how to price things justly, based on tradition/custom/power/etc rather than economic principles alone. it's this isolation of 'the economic' as operating in accordance with its own internal logic that represents the shift toward value-relationships which i think characterizes capitalism.
so when marx asks 'why does labor take the form of value', we can read the question as asking why a transhistorical physiological function (labor-as-such) begets this historically specific property (value), but also/inversely, why does this peculiar social object (labor in the one-sided economic sense) somehow posit an objective regulatory mechanism which structures/aggregates our own subjective estimation of things (based on notions of sentiment, tradition, etc which become outmoded or subsumed by capital but are not essential to it). if you only ever read the question in the first way, then you'll get hung up on the mere existence of pre-capitalist exchange and also potentially project value-constituting labor back onto all of history because it's reduced to a purely physiological act, devoid of a social context/content.