Sure, some of the things you mention are "better" or at least otherwise accomplished using event-triggered Lambdas. But there's plenty of reasons why you might want to run a job that takes under 5 minutes on a schedule.
For instance, daily or weekly "digest" emails. Or a nightly stats job, where expensive-to-calculate yet not-immediately-critical queries are performed and the results stored or exported. These are things where you want them to happen at a certain time. Cron does that.
It's possible that customers didn't understand why AWS thought Lambdas were there, but that's irrelevant. Suddenly, Lambdas were there, and people thought of more use cases (SQS triggering, connection to API Gateway, etc) than AWS had implemented from the start. That's not the customers' fault for being creative.