Why do people say they live on a fixed income?

Heard someone talking about senior citizens and said they live on fixed incomes. Don’t most of us who are salary live on a fixed income? I have no guaranteed raise annually, meanwhile social security does have a cost of living increase so wouldn’t I be the one on a fixed income?

Unless you are hourly and have the OT opportunity, you make the same amount every check with no real way to increase that.