It’s complicated 
It’s not the airlines, per se. Sure, if you book via United’s website, you obviously will only find United flights and those of their closest partners, wit results sorted in whatever way pleases United.
But if you book through travel agents, who in turn would usually be connected to a GDS, there are very clear rules on how results are sorted (by default). This is one (if not the only) reason why codeshare exists: Connections “of a single airline” are always sorted above mixed ones.
In current-gen ORS, I almost certainly won’t attempt to address this issue. But there are several individual components to a solution on the roadmap for the DS (Codeshare, GDS/BSPs, Distribution Channels, to name a few).