You still need extra code to check that the format variant matches your expectations. I guess you could go very bare-bones and just assume that it’s in the right format and only check for buffer overflows, at the cost that errors will be harder to diagnose when the format is wrong. And you’d still have the overhead of the larger metadata structures of TGA.

The fact is, none of the existing formats is a perfect fit for the embedded use-case here. It makes sense to introduce a new, simple and straightforward format if it addresses certain typical use-cases better than existing formats do.