I don't understand why you don't understand how RNG works.

If I had 7 marbles and put them in a bag and picked one at random. I have 1/7 chance of picking up a specific marble. I put the marble back into the bag. It will always be 1/7. If I go into a dungeon and open a chest there's a 1/7 chance it will be either one of those 1 sets. It will always be 1/7.

Unless your complaint is literally there are 5 sets that are 'DPS' and for some reason think that the DPS will automatically get them when you can roll on them yourself, unless one of those sets is something that can be needed upon; which is STILL a 1/7 chance that a DPS will be able to do that.