I know that it is common in DC regulators to use both a high capacitance electrolytic capacitor and a low capacitance ceramic capacitor in a parallel, in order to maintain sufficent capacitance over a wider frequency range.
Why are not these parts sold in a single package? Presumably a capacitor combining electrolytic and ceramic dielectrics would be a closer approximation to an ideal capacitor.
I'd say it's probably cheaper to manufacture them as two separate processes. No two designs are the same so it would be difficult to come up with value combinations that people would use.
More importantly in decoupling inductance is very important especially as you go up in frequency. So I prefer a tiny surface mount ceramic with minimum loop area for my high frequency decoupling. But I can use an electrolytic's larger package for my lower frequency bulk supply needs.
Stuffing my ceramic cap up in the electrolytic's package would negate the point of using it.
