Below cutoff frequency, there is no energy flowing in to the waveguide. And as there is no energy flowing, no losses are needed. You only see so called evanescent waves that decay exponentially when going inside the waveguide.

https://en.wikipedia.org/wiki/Evanescent_field

Below cutoff frequency, there is no energy flowing in to the waveguide. Everything you try to feed in is reflected back. And as there is no energy flowing, no losses are needed. You only see so called evanescent waves that decay exponentially when going inside the waveguide.

https://en.wikipedia.org/wiki/Evanescent_field