It's not that the starches have already been converted. The kilning process kills the enzymes that do the conversion. So the only way to extract sugar is to mash them with a base malt that still has the enzymes. If you steep these crystal or roasted malts on their own you will only get flavor, aroma and color. Still beneficial, but not fermentable.
I think you are wrong. But I also think I was wrong.
The kilning process does not convert the starches in crystal malts to sugars, as I said. According to How To Brew, crystal malts undergo a heating process during their malting that allow the enzymes in the grain to convert the starches into sugars. The kilning process caramelizes those sugars. Thats what give them color and makes it so they don't have to be mashed.
If, as you say, there were starches in the crystal malts, you wouldn't want to steep them - as they would produce a starchy beer.
In the end, there isn't any conversion of crystal malts happening in the mash.
From how to brew:
Besides the lighter-colored base and toasted malts, there is another group of malts that don't need to be mashed and these are often referred to as "specialty malts". They are used for flavoring and have no diastatic power whatsoever.
Some of these malts have undergone special heating processes in which the starches are converted to sugars by heat and moisture right inside the hull. As a result, these malts contain more complex sugars, some of which do not ferment, leaving a pleasant caramel-like sweetness. These pre-converted malts (called caramel or crystal malts) are available in different roasts or colors (denoted by the color unit Lovibond), each having a different degree of fermentability and characteristic sweetness (e.g. Crystal 40, Crystal 60). Also within the specialty malt group are the roasted malts. These malts have had their sugars charred by roasting at high temperatures, giving them a deep red/brown or black color (e.g. Black Patent malt). The Lovibond color scale ranges from 1 to 600. See Figure 70. To put this in perspective, most American mega-brewed light lager beers are less than 5 Lovibond. Guinness Extra Stout on the other hand, is comfortably in the 100s. Specialty malts do not need to be mashed, and can simply be steeped in hot water to release their character. These grains are very useful to the extract brewer, making it easy to increase the complexity of the wort without much effort.
