The point of partner airlines (as opposed to alliance partners - common membership of an alliance comes with additional responsibilities and obligations) is purely to provide feed to United. United partner with airlines that can supply them with additional passengers that will end up traveling on United services at some point during their journey. As such, because you are effectively a United passenger, you have a valid expectation to have United status perks (if any) recognised - at least to *some* extent - while travelling on the partner airline(s) as part of a larger United ticket.

United partner with Aer Lingus on select itineraries that involve transatlantic travel, and which are purchased directly from United (or from a third party, but which must also involve travel on and with United flights on the same ticket/itinerary).

United has no interest in passengers that are neither connecting to or from their own services.

The scope of the partnership simply doesn't extend to United passengers who arrange their own travel on Aer Lingus; that sort of travel provides no income or revenue to United, and therefore they have no interest in having "their" passengers recognised as such when doing their own thing with any such airline on tickets that fall clearly outside the scope of the partnership. Neither does Aer Lingus, as the provision of any such perks/bonus comes at a cost, and that cost is not going to be met by United.