There's a simple reason why - it costs x to certify 1 developer, and y to certify a piece of software.
For most software, "y" is still too expensive to offset the potential damage caused by software failure (compared to a jetliner, where the potential cost is huge).
A programmer may create hundreds or even thousands of pieces of software during a career. Average x over that number of projects, and it's negligible. Multiply that number by y, and you have an enormous figure.
Also, the cost of certifying a piece of software to provide any kind of assurances of quality is time consuming and expensive. Compare this to a certification that shows that anyone who wants to claim to be a "software engineer" actually knows the first thing about software engineering, and is at least capable of producing software in a systematic way.
It's just like we license drivers once and assume that that basic level of competency will ensure "quality driving" on a variety of road types. We don't certify each driver for each road individually.