Says who? Because the modern computer is basically an appliance, or fast becoming one. As such, companies and their brands are on the hook for making the experience, usage, and support of their products as seamlessly usable as possible.
You can certainly go other routes with alternative OSes or even open source and not bother with this. That's your choice. Just as you can build your own appliances of other kinds of you wanted to. But if you're buying a TV or refrigerator, what it is is what it is. It's clear that average people would rather see their computers that way in terms of product brand and how they use it. They don't want what they see as complexity in it at all. As such, the company brand is responsible. Also, your description simplifies it too much, there are user-facing issues that would seem easy to you or I but complex to the average user or malware that need to be addressed so that you can't have it all in terms of blocking some but letting others through unless there's a control for that.
Couple comments/links to read:
[gizmodo.com]
[blog.wilshipley.com]