That's because there are way too many variables to calculate "damage per minute" accurately:
How many monsters are around you and in which range?
How many players are in your party?
What skills and passives, especially group support skills, are your party members using?
Which items are your group members using that might impact the group's DPS (Sever?)
What kind of monsters are you fighting: A Tremor in defense stance with lots of damage reduction? Elites with CC abilities that limit your effective DPS? A group of illusionist mobs that burst your DPS for a short amount of time? Just white mobs?
There are literally millions of synergies that you'd have to account for. All that people do right now is run Ghom tests - it's one single mob versus one single player, and still serves as basis for discussion as to how valid this result is. Imagine you had a DPS meter like in WoW, you'd do one complete rift, and you'd look at the DPS curve. It would be like a very spiky graph plot. Which segment of this graph would you call your average DPS? Just the average over time? What does it show you? It's exactly what people already calculate - average rift clear time. A Conduit Pylon ruins your entire results, though.
Don't get me wrong, I'd be more than happy if such a tool exists. If you wanna go for it, absolutely by all means go ahead. But as someone who only took small segments of this "simple overall tool" I can tell you that even implementing 1% of the functionality you want can take forever, and not just talking about interface here (of course, a good interface takes lots of time and testing). I just want to give you an honest heads-up that you are drastically underestimating the complexity of your project.
Oh yeah, this would take long to make, trust me ;-) I've done smaller projects revolving around D3 calculators and the devil is in the detail. There's lots of hidden mechanics and caveats to look out for, so this is not something you can do over a weekend.
There are lots of websites that offer those numbers though:
The problem is that the game's API is still not completely fixed and therefore many of the bugs on those tools will only be working 100% if Blizzard fixes the API first.
The only way to have 100% accurate data is to enter them manually, such as in Loroese's spreadsheet.
That's because there are way too many variables to calculate "damage per minute" accurately:
There are literally millions of synergies that you'd have to account for. All that people do right now is run Ghom tests - it's one single mob versus one single player, and still serves as basis for discussion as to how valid this result is. Imagine you had a DPS meter like in WoW, you'd do one complete rift, and you'd look at the DPS curve. It would be like a very spiky graph plot. Which segment of this graph would you call your average DPS? Just the average over time? What does it show you? It's exactly what people already calculate - average rift clear time. A Conduit Pylon ruins your entire results, though.
Don't get me wrong, I'd be more than happy if such a tool exists. If you wanna go for it, absolutely by all means go ahead. But as someone who only took small segments of this "simple overall tool" I can tell you that even implementing 1% of the functionality you want can take forever, and not just talking about interface here (of course, a good interface takes lots of time and testing). I just want to give you an honest heads-up that you are drastically underestimating the complexity of your project.
There are lots of websites that offer those numbers though:
http://www.diablo3tracker.com/
http://www.diabloprogress.com/
http://d3up.com/
The problem is that the game's API is still not completely fixed and therefore many of the bugs on those tools will only be working 100% if Blizzard fixes the API first.
The only way to have 100% accurate data is to enter them manually, such as in Loroese's spreadsheet.