The simplest answer to this imho is - each situation is different so what dps calculation do you want displayed? Yes, the game calculates this on the fly for individual hits / dots during the game based on 50 different factors at that split second in time… but how do you want this information displayed to you in a usable way that is actually accurate enough for you to do something with the information?
For example, attacking the dummy (which has zero modifiers enabled) might say that with skill X, you are doing 10 dps. Attacking the same dummy with skill Y, you are doing 20 dps…
Now factor into this, all the different active skills you can have, auras, enemy debuffs, buffs, passive procs etc… skill x now does 10 to 100 dps and skill y does 20 to 200dps…
Add things that are random and time based, like bleeds, poisons, gear procs, random skill procs… now skill X does 100 to 2500 dps and skill y 200 to 20 000 dps at any given moment…
and this is just for the dummy… and doesnt include map modifiers, mob modifiers, debuffs on your character or anything else that would be active in normal play.
yes, the game calculates this actively during play, but how are you going to use this information while playing? Hit damage calculations are already there but its hard to show the actual dps on a skill when there are so many other inputs that are dynamic.
so the question is no longer as simple as showing a dps figure - sure, the devs can show a rough dps figure right now - but is it going to be accurate? and if its not accurate, then whats the point?
take a build that does 500k crits on the dummy at level 100… during empowered mono runs, it does 20k crits on one rare and 40k crits on another… which dps do you want to see?
Yes… they will eventually figure something out, but its hard to SHOW it accurately…