Because they put assembly in countries with cheap labor, and optimize the assembly process so that most workers don't have to perform any complex operations, or any operations that involve thinking.

Things like PCB manufacturing? Putting those BGA chips where they go? Done entirely by machines.

Now, a notable exception to this rule is the "rework" or "remanufacturing" lines - where actual human specialists take devices that failed QC, or used devices, diagnose them, and bring them up to standard.

Those can be very involved. But official manufacturing still has strict limits on how far are they willing to go - and unofficial refurbishment lines have them beat on repair complexity.