Why are parasitic capacitances the main factor for limiting high frequency performance of active devices such as transistors?
Most of our circuits operate on voltage signals and voltage cannot change instantaneously across a capacitor coming from the equation that describes a capacitor \$I = C\frac{dv}{dt}\$. The \$\frac{dv}{dt}\$ is a derivative of voltage which means the voltage must be continuous which means it must be a finite, smooth, gradual slope. And any time you have two conductors separated by an insulator, you have capacitance. As you can imagine, that happens a lot.
Very similar to why does the mass of something limit how quickly it can vibrate. Because more mass = more inertia which means more energy to force it to change direction. Sure, you can make it faster by adding more energy to make it change faster, but if you keep adding energy, eventually something is going to give and damage will occur.