@Sunny2 ‘First of all, being “kept” assumes no marriage between the couple,’ I disagree.
As @JLeslie mentions “it has to do with power”. If the breadwinning and/or other aspects in a marriage are greatly lopsided, then one of the couple is kept.
‘If the couple is married, it’s agreed they will jointly decide how their finances will work.’ That’s an ideal, but certainly often not the case, especially when one of them dominates the decision-making with the de-facto power of the pocketbook.
There is also an aspect of being kept that is the submission to the other, being passive and in a real sense unresponsible about a lot of day to day responsibilities (it’s not irresponsible because both parties are okay with it).