This isn't currently allowed because it's not necessarily a good idea. The way versions work in Salesforce is that each version installed is tracked by the platform. Metadata that depends on a version that was never installed will fail to deploy. This creates a big cat-and-mouse type problem for subscribers of these packages.
To minimize this disruption, you're only allowed to automatically increment build numbers, which do not affect metadata dependencies. Even if you could do this, I would recommend that you don't. It may be a convenience for you, but it may artificially introduce extra work needed to update orgs with new versions.