Why is a fever positive feedback?

A fever is not a positive feedback loop. It is a negative feedback loop.

In a negative feedback loop, the output of a system acts to reduce the input. In the case of a fever, the increased body temperature (the output) causes the body to produce more heat (the input). However, this increase in heat production is not sustained. The body eventually reaches a new equilibrium temperature at which the heat loss matches the heat production.

In a positive feedback loop, the output of a system acts to increase the input. This can lead to a runaway reaction, in which the output increases exponentially. There are no known positive feedback loops in the human body that are involved in temperature regulation.