Why are AeroMexico one-way flights so expensive??

My family is planning to visit from New York in February- the plan is to visit Vegas for a few days then take the new AeroMexico flight to spend a few days in Peñasco.

The round trip flight (Thu-Sun) comes out at $111+$166=$277 total. Sounded perfect... BUT... the Vegas flight gets in only 1:45 before the last flight back to NY, and they have to work the next day so can't do an extra night in Vegas/ red-eye flight/ etc.

So I thought that instead they could take the Head Out shuttle back to Phoenix a bit earlier Sunday and take a return flight from there. Also sounded perfect... BUT... I checked AeroMexico, and if I choose just a one-way to Peñasco on Thursday, it comes out to $260. So essentially one-way costs as much as round-trip???

I imagine since its just 1 flight per week they want to "encourage" everyone to take the round trip but c'mon that seems excessive doesn't it? In the US almost every major airline abandoned this model years ago, and now every flight is priced as an independent segment. Does anyone have anything to add here that I'm missing? Any way around it? Thanks!
 
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