First, it is the partner that first determines availability, not UA. And partners may limit how their segments are combined with other of their segments (one way is married segments). This including requiring or restricting how segments are stringed together.

And UA will not allow you to individually select segments and string them together -- include the flight operator restriction. It has to come up in the search naturally.

Best not to try to apply logic (or using distance or cost as the metric) to understand airline bookings. There are things unknown to you and us impacting the choices.