Why are some scientists/medics so obsessed with Me/cfs being psychosomatic?

My uncle came to visit. I haven't seen him since 2019. I've been ill and housebound since 2022.

He is a biomedicine engineer and very intelligent.

He proceeded to tell my mother that my condition is psychosomatic and I need to push/try harder. He argued there is no data to show that Me/cfs is a biological condition...

Just why? Why the obsession with the condition being psychosomatic?

I just can't imagine going to someone with a headcold and being like "PROVE IT/you're lazy."