A key difference is that software is already covered by copyright. Identical copies violate copyright for software. For a mechanical device, identical copies generally don't violate copyright (trademarks, text, etc aside).

Mechanical devices need patents for the design, as a competitor can easily re-implement an absolutely identical design. It's often clear by inspection how it's made. A different device that accomplishes the same task in a slightly different way doesn't infringe on the patent.

Software already has protection against identical copies through copyright. The key difference is that software patents go farther; they claim to cover the concept. Any implementation of the concept violates the patent. Re-implementing software to accomplish the same mathematical concept in a different way still violates the patent.