The top skaters are getting level 3s and 4s because everyone has to in order to get those points. If it was a fixed level with just GOE, you can bet top skaters would reduce the difficulty of their other elements. Lower skaters would have to keep their difficulty in order to be on par with the top level skaters. A low-difficulty element performed by a top skater can score as much as a high-difficulty element from an "inferior" skater. Also, top skaters will be deducted far less severely than inferior skaters (we already see that with top skaters getting -1's and -2's when they fall; the same courtesy isn't extended to lesser skaters even if those skaters met all the other GOE requirements of the jump).