tbh, when MNers who make OP's claim are asked for specifics, they usually come up with a long list of foods their child does eat. Only not as long a list as PP would like...
A local lad (he's 19yo now, I saw him on a bike ride with his mum recently) got to his teen years eating precisely & only 5 foods: 2 reasonably healthy drinks, one type of crisps, 2 types of sweet. He probably still consumes only these. And water. I think.
He was always the nicest lad, sensible, sporty & his mum the most laid back person I met, but he wasn't budging & she wasn't going to lose her mind over it. Far from wasting away, He tended towards being pudgy.
Tell us how old your child is, & what they actually ate at least once this week, OP. My own fussy eater has finally pretty much outgrown his fussiness. It took being a ravenous sporty teenage boy to get him there, though.