Why are carnivores subject to biological magnification?

Because they are higher up in the food chain.
In order to fully unpack this question, we must first understand what biomagnification is.
In essence, biomagnification is the process in which biological matter from organisms accumulate as they are consumed by predators at each succeeding level in the food pyramid.
Sounds complicated, but here is an example. We place pesticides on crops to ensure they are kept safe from rodents and pests. One such pesticide, which we’ll call DDT, can be placed on corn.
Lets say rats eat the corn. Keep in mind, one rat can only eat so much corn – so lets imagine the amount that a single rat eats is defined by ‘x’.
Snakes eat rats. Lets say, for the sake of simplicity, there are two snakes in this field, and they eats 2 rats each. Even though a single rat only consumed ‘x’ amount of food (as well as the pesticide on the food), the snakes ate two rats each, so technically, a singe snake consumed ‘2x’ more than a single rat. That is, the snake consumed ‘2x’ amount of pesticide as well.
Eagles eat snakes. Lets say one eagle saw both snakes, and since it was extremely hungry, ate both snakes. The eagle has now, in addition to helping explain the process of biomagnification, has consumed a total of ‘4x’ amount of pesticides (each 2x from each snake).
The result was fatal, since the pesticide DDT was linked to thinning the egg shells of Eagle offspring and subsequently decreased their birth rate. Even though a single rat only ate a small amount, its the fact that many rats are eaten by one snake, and many snakes are eaten by one bird – the levels of toxicity (in this case, DDT) magnify as the trophic levels increase.
Hope this helps!