Your average Microsoft error “error 37253” is worthless to me
This is a security thing. A descriptive error message is useful for troubleshooting, but an error message that is useful enough can also give away information about architecture (especially if the application uses remote resources). Instead, provide an error code and have the user contact support to look up what the error means, and support can walk the user through troubleshooting without revealing architecture info.
Another reason can be i18n/l10n: Instead of keeping translations for thousands of error messages, you just need to translate “An Error Has Occurred: {errnum}”