Like @Miss_B already said there are many reasons. Distributing files via the Markerplace has it's pros and cons - for developers AND users.
I prefer not to offer all files in the MP, only themes. That's my decision and you have to respect that. In the end it's simple, If you don't like that just don't use the addons, I don't want to sell anything.
Funfact: On my old XenForo site you can instant download all my addons and translations without signing up. The difference is they are abandoned after I left XF. They are no longer developed and no longer receive updates. If anyone still wants some of the old stuff he can get it, but on his own risk because I don't support them anymore.