It's not standard because people frequently do things like type a credit card number into a form on a public computer. Lots of potential for privacy breaches.

With and option to disable this behavior from the HTML websites could chose to disable it on forms which might contain sensitive data. Browsers could even disable it by default and let the websites enable using an attribute.

Even on a private computer, having it stored in a file can be bad, because if another process gets on there and reads the storage, it'll get the data.