Often skaters will lose points if they don't get the levels they were aiming for on the spins or steps, or the rotation on the jumps (< or << calls, doubling out or popping), even without visible errors, although obviously they lose points for those as well. Or, of course, leaving stuff out or having whole elements disallowed because they don't fit the rules for some reason.
But they can also get extra points in positive GOE for well-executed elements. So it's useful to be able to point out where the skater made up for mistakes with high quality elsewhere.
The system is already designed to allow planned base value to be announced before the performance. Using a "maximum" technical score instead would require each skater to pretend to expect not only all level 4s but also +3 GOE from all judges on all elements, which means no one would ever come close to their maximum. Well, maybe Davis/White and Virtue/Moir.
Anything else based on maximums would mean redesigning the whole system.