Did it ever occur to you that they don't just have maintenance personnel at every airport? Because what I'm saying is that no airline in the world has maintenance personnel at every airport.
Spirit, Frontier and Allegiant are Airbus only and would require an Airbus tech. Airbus planes are pretty decent on that the A19-A321 planes are pretty much exactly the same in parts and configuration except that some are longer and or wider than others. On the other side of things Southwest has only Boeing planes, mostly 737 and 747.
Pretty much every other airline has a mix of different planes (Boeing, Airbus, and Bombardier, Embraer). To do what you're talking about every airline that flies more than one plane would have to have a technician for each of those plane types on the ground at every airport they fly to. That's 5000 airports, with at least two technicians per airport (assuming they only have one flight in and out of there at a time which is ludicrous). The average number of flights going in and out of any one airport at a time. Daily there are about 45,000 flights per day per FAA statistics not including private flights.
At Delta's hub in Atlanta, there are around 2100-2700 flights per day. Delta says they have about 6,400 AMT's worldwide One singular airport out of 242 airports that Delta flies to. 24 hours a day for most airports. They would be required to keep at least 8 people per airport per average number of flights leaving or arriving per at the same time. Let's say that at their hub they only have 5 planes on the ground at any given time ( a gross miscalculation of how many planes fly into their hub, but the math is cleaner). Delta has 4 different plane manufacturers's planes in their fleet. That's 4 mechanics on an 8 or 12 hour shift multiplied by 5 planes let's say per average turn around time of 30 minutes. You'd need 20 techs
At every single solitary airport Delta flies to. Per shift. Supplied by the airline. It's a logistical nightmare and this number balloons when you realise just how.many departures and arrivals there are and at what intervals at pretty much any major airport. 9,640 AMT's assuming 12 hour shifts. Just for domestic USA flights, not including planes that are down for maintenance outside regular maintenance schedule. When the fleet only emplyes 6,400 AMT's world wide.
I cannot stress this enough, but you're making a lot of assumptions here. And you don't think it's an unrealistic expectation specifically because you have no idea how any of this works.