Why Film capacitors use low-loss plastic dielectrics (polypropylene, polyester, polystyrene) whose loss tangent stays very small well into the RF range, so the dielectric is not what limits them. What does degrade is the metal: the thin foil or metallized electrodes and the leads carry current only in a shallow outer layer as frequency rises, because skin depth falls off as 1/sqrt(f). That crowding raises the effective series resistance, and the I-squared-R heating in the electrodes and leads becomes the dominant loss mechanism.
Watch out Dielectric loss is the tempting pick because it dominates in many other capacitor types, but the plastic films used here have exceptionally low loss tangents, leaving conductor resistance as the main culprit.
Film dielectric = low loss; the metal is the problem. Skin effect raises ESR as sqrt(f).