I think it is this way because back-end development requires more expertise than GUI development. Steady more experienced developers are better suited than new hires.
Slow = bad
Fast = good
They probably think a good GUI is a question of taste and can't be measured and they don't want depend on "luck"
In either area (frontend / backend) the complexity depends entirely on the problem that you are trying to solve and I am sure there are plenty of FE problems that require a high level of expertise to solve elegantly.