Kent,
I do agree that in an ideal world, all vehicles should perform up to their spec limits, but this isn't an ideal world. First off, about 75-80% of compact trucks never pull anything and those that do pull only pull light. Not a slam, just a keen observation. The ramification of that observation is that automotive engineers (being a bright bunch) design the compact trucks to have very "car like" ride and handling. It's a major buying point. "That one rides like an old wagon, this one rides nice!" Ever heard that? You can bet the manufacturers have and responded to it. So, as a general class, compact trucks have pretty light suspension, brakes, steering and transmissions. Why? It's cheaper to make, gets better mileage but most of all that's what the buyers want. Small, light, nimble with car like performance. The focus is not on hauling performance, sure they can still do it, but it is about 5th or 6th out of 5 design priorities.
Next, "properly equipped" usually means that you checked off a bunch of boxes at the time the truck was ordered. Better tires, wheels, front suspension, rear suspension, bigger radiator, additional transmission cooler, hitch, wiring, HD electronics to accept the load of trailer lights, etc. AND using a weight distributing hitch, a trailer with brakes having the load properly distributed. Most importantly the 3.92 rpm screamer gear ratio. I've seen light trucks squat like a dog tak'n a cr-p because the load was too far forward by as few as 5-6 inches. And that's a load within spec ratings. Don't forget the specs are usually written to make the mfg look good and are for the lightest version they make. 2wd, 2 door stripped model. There is less capacity by adding options like club cab, 4wd.
From the Dodge web page, the 4x2 St Dakota weighs 4411. Max towing is 7000. Great right? Read the rest. Standard towing is 2000, which version did you buy? There's more. Gross vehicle weight is 6010#. That's gross and includes you, your passengers, gear and trailer tongue weight. So, if you weigh 200#, wife is 150#, 2 kids at 75# each, misc gear at 200# what's left for the tongue weight? 6010-4411-700= 899# max tongue. But there's more. Did you get the club cab? 4x4? have a full tank of fuel? As you can see, the max vehicle load limit quickly eats up the budget for trailer tongue weight. It also eats up some of the max tow capacity as there is a Combined Max weight limit too. Max Combined is 11700 - 4411 = 7289 max trailer. So, the realistic max is closer to 5500-6000# and you probably have to take great pains to balance the load to limit tongue weight.
Now, the great big trucks with diesels that are rated to tow 13,000#, are they immune to this same stuff? No way! But, they have more available budget. The realistic max is probably 11,500 if the listed max is 13,000. (and note, the max is not 15,000, it's around 13,000 for the diesel dodge 2500)
Remember, if you didn't order it special, the max towing capacity is around 2,000 pounds. IF you did order it special, the max towing is 7000 pounds MINUS the weight of all the stuff in the truck including fuel weight, option weight and people weight. And that's only if you don't exceed the max vehicle weight due to tongue weight (which should be about 12-15% of the trailer weight).