Because the game designers wanted the DM to have more control over these types of situations. A codified set of DCs might work very well for your game, but not for mine. A low fantasy game might only allow a few feet extra, with fairly difficult DCs (something like DC: 10/15/20 for +1/2/3 feet for a long jump), because it's a grittier type game. A high fantasy game may allow up to double your jump distance with fairly low DCs, because it's more about epic situations.I am not sure what you mean with the inclusion of "in some circumstances." It is pretty plain in the Strength entry in the PHB that one of the things you can do with Athletics is "jump an unusually long distance." That is a statement of the rules. What I am wondering is why there are no benchmarks for that action.
If you truly have an issue with it, then make your own chart. Figure out how much extra you want to allow, and how hard. This will keep you consistent, and solve your problem for the future.