Audio files aren't owned by the creator they are owned by the label, so the label chooses to release stems every now and then as a remix contest to promote the song.
But the ACTUAL work done in the project is owned by whoever did the work. Not the actual audio, but the meta data/workflow that went into making the audio. If a mix engineer mixes a song, they own everything they did and all their work on that stem. Not the audio, just the work that went into it(processing chains, etc..). So this is why you aren't going to see a lot of project files for bigger songs out there. The mixing engineer or whoever worked on it doesn't care enough or have any motivation to release the project files. The reason you see the audio stems is because the label has a reason to release them. The label couldn't release the project files if they wanted to though.
This goes out the window obviously if everything was done by one person and no label was involved. But if you're looking for stems and project files I'm assuming its for a commercial song that was released through a label. Stems=Labels.. Project data/work that goes into the stems=whoever did it. Only way to get project files would be to contact the composer/mixer directly. And if the songs owned by a label its a long shot you'll ever get the project files because of something or other the creator of the actual work had to sign. Plus i doubt anyone getting paid to engineer is going to release their workflow to the public, for anyone to piece together and replicate. That would be like giving out your secret recipe.
