According to this table, an untrained 97 pound adult woman is expected to be able to lift a 31 pound barbell over her head.

On some airlines, the untrained 97 pound adult woman appears to describe most of the flight attendants. Of course, the table lists one rep maximums of weights in convenient barbell shapes, not possibly awkward or unbalanced carry-on baggage. And a flight attendant might be asked to assist more than once.

So if the airline does not want its flight attendants to be put in a position of refusing to help passengers with their carry-on baggage (looks bad from a customer service standpoint), it may impose a 7kg (about 15 pound) weight limit so that the carry-on bags are well within the ability of the flight attendants to assist.