The problem was, she didn’t tell anyone where that will was.
When she died, no one could find the will. It’s entirely possible that it’s still sitting in a lawyer’s office, or that she locked it away in a safety deposit box at a bank she used for nothing else. And in the end, after a years-long fight between her husband and her mother, a third party was appointed as the executor.
It doesn’t matter whether you’ve made a will if your executor and beneficiaries can’t find it.