UA and DL don't have interactive sell capabilities at least as of last fall (and likely longer then that as DL was frequently failing to provide a confirmation number well before that, it was just in the fall I'm told they were formally taken off the list of airlines UA recognizes holding interactive sell with). If DL's system doesn't provide a confirmation number back to UA, which it rarely does on domestic itinerary rebooking, it requires calling DL to manually verify the space and have them build the PNR/attach the eTicket,

You think UA agents are tough to deal with? Go look through some of the DL discussions about their phone support (starting with this recent discussion over the past 3 days Why is reissuing a ticket with DL worse than purgatory?) and that is what UA has to deal with when rebooking on DL.