They do, but compatibility isn't perfect and fixing anything requires huge reverse engineering knowledge or prior knowledge.
See:
"... PSD is not a good format. PSD is not even a bad format. Calling it such would be an insult to other bad formats...
If there are two different ways of doing something, PSD will do both, in different places. It will then make up three more ways no sane human would think of, and do those. PSD makes inconsistency an art form...
Earlier, I tried to get a hold of the latest specs for the PSD file format. To do this, I had to apply to them for permission to apply to them to have them consider sending me this sacred tome. This would have involved faxing them a copy of some document or other, probably signed in blood. I can only imagine that they make this process so difficult because they are intensely ashamed of having created this abomination..."
See:
"... PSD is not a good format. PSD is not even a bad format. Calling it such would be an insult to other bad formats...
If there are two different ways of doing something, PSD will do both, in different places. It will then make up three more ways no sane human would think of, and do those. PSD makes inconsistency an art form...
Earlier, I tried to get a hold of the latest specs for the PSD file format. To do this, I had to apply to them for permission to apply to them to have them consider sending me this sacred tome. This would have involved faxing them a copy of some document or other, probably signed in blood. I can only imagine that they make this process so difficult because they are intensely ashamed of having created this abomination..."
https://news.ycombinator.com/item?id=575122
https://layervault.tumblr.com/post/56891876898/psdrb
https://news.ycombinator.com/item?id=6129237