In reality, it’s likely that it’s because Minecraft is largely targeted to kids. The primary presented idea of Minecraft is a game that is appropriate for kids. Web browsers don’t make the same kind of presentation–their purpose is to render websites, but don’t otherwise promise anything regarding the content viewed.

It would be a bit similar to a department store that stocked knives in the kids’ toy section. Selling knives isn’t really problematic, it’s where they’re located and the implied intent of that section.