However, the "detect" DC of a trap is always signficantly lower than the "disarm" DC of a trap. Moreover, you roll your detect rolls constantly, so if you are actively searching and have maxed your search, eventually the level 20 will spot the trap set by the level 15 (probably very quickly, I might add). The reverse is not necessarily true -- a trap set by a level 20 is going to be much harder for a level 15 to detect.
Assume both characters have maxed out their search skill at their respective levels. The level 20 has a 5 point advantage in search, as well as a 5 point advantage in set trap. All the default detect DCs are eiher 10+set trap skill or 14+set trap skill (not counting epic traps, which I havae never seen here). These DCs are pretty low, so the real difference maker is the set trap skill of the setter. Here, the level 20 has the clear advantage -- their trap is going to be 5 points higher to detect, and the level 15's search skill is 5 points lower, so there is a net 10 point difference between a level 15 trying to detect a trap set by a 20, versus a level 20 trying to detect a trap set by a level 15.
But back to the original issue -- the notion that a level 20 setter versus a level 20 disarmer should be a 50/50 proposition is a false equivalence. There is no reason why that should be a 50/50 proposition -- if a trap is nearly impossible to disarm, then it is nearly impossible to disarm for everyone. Idon't see why the two skills have to "balance" each other.
This is different from two "levels" balancing each other. In your example, the two level 20s should be equivalent in skill -- they can both set traps at an equivalent level, and disarm traps at an equivalent level. The same would go for two level 15s. The characters are balanced in what they can all do. The concern you are raising is that there are traps that are too powerful, such that they are undisarmable by anyone beyond a certain point.
Take the deadly traps out of the equation, and focus on minor traps for a moment, and you will see my point: a minor spike trap has a base detect DC of 10 and a base disarm DC of 22. If set by a maxed out level 20, you are looking at a base DC of approximately 39 for detect, and a base DC of 47 for disarm (depending on how the DEX and INT of the character was built, but assuming an end DEX of 23 and INT of 14). This could all go up depending on gear, feats, buffs, etc., but these are the likely base numbers. Feats, gear, buffs, etc. are all equally available to everyone, so those effects can be discounted for the moment, since the core number that is critical is the base number.
A level 20 disarm-build searching for this trap will have a max search of approximately 25 (assuming an INT of 14). Thus, rolling every round, this character will have to roll a 14 or better to spot the trap (35% chance of success each round). Thus, it is only a matter of time before it gets spotted. The same trap set by a level 15 should have a DC at least 5 points lower, making detection success occur at a 9 or better (60% of success each round). The level 20 will likely not ever by taken by surprise by a trap set by a level 15, unless he or she is just running around without any precaution.
Conversely, it will be much harder to disarm this same trap. The level 20 disarm build will likely have a base disable trap skill of approximately 25 (assuming an INT of 14). Thus, without buffs, even with a take-20 this character would not be able to disarm a trap set by another level 20. However, the same trap set by a level 15 will be 5 points easier (for an approximate base DC of 42), meaning that the level 20 will be able to disarm that trap with a take-20.
Thus, when you strip away all the buffs, feats, and magic items and focus instead on the innate skills of the characters, there is no "imbalance" between level 20s and level 15s. However, more interestingly, you can see that a skills battle between a level 20 and another level 20 is NOT a 50/50 proposition -- the trap setter has a few points of advantage over the disarmer, making the trap undisarmable. Nevertheless, it should be pointed out that the trap is still detectable, and it can also be easily examined (base DC -7), and can be flagged so that others can avoid it (base DC -5). Players should use "examine" and "flag" far more often than they do, and recognize that walking around a trap is often the wiser course of action.
If people believe that "balance" is achieved by a disarmer being able to disable a trap, as opposed to being able to spot and flag a trap and avoid it entirely, then I think this is a mistaken impression. As long as you can find the trap, you can avoid the trap. Even if you can't disarm it yourself and feel an absolute need to get rid of the trap entirely, you can simply invest a pocket full of Summon I scrolls and have your badger disarm it the hard way (that is essentially what modern bomb-disposal squads do today with their robots).