A machine on a factory floor wears. Software does not. The file you paid for in 2022 is byte for byte the same file today, and that is exactly the problem. It was written to fit a world that has since moved on without it.
That drift is invisible until it is not. A security hole is found in a library your application uses, and there is no patch for you because the version you are on stopped being supported last year. An app stops appearing in the store because it targets an Android version Google no longer accepts. A sync to your accounting system dies overnight because the platform retired the connection it used, six months after announcing it in a notice nobody read.
None of those are failures of the original build. They are the predictable result of leaving a system alone. And every one of them is cheaper to handle in advance than on the morning it lands, which is the entire argument for maintenance.