Why does cancer change someone’s appearance so much ?

It typically doesn't.   Most cancers are invisible. 

However,  most treatment is VERY hard on the body,  and a stressed liver shows in many ways. 

It's usually the treatment that gets you.

This varies from patient to patient.

Some have very little alteration.

Some treatments cause scarring.  Some cause nausea & weight loss. Some trigger weight gain.

Some trigger hair loss.

Some leave a patient looking tired, which can be ageing.

Some patients deal with constant pain.