Browsers are sandboxed, and working on the web browsers themselves is a very small niche, as is working on kernels.

Software increasingly runs either on dedicated infrastructure or virtual ones; in those cases there isn't really a case where you need to worry about software running on the same host trying to access the data.

Sure, it's useful to have some restrictions in place to track what needs access to what resource, but in practice they can always be circumvented for debugging or convenience of development.

Browsers are sandboxed by the kernel, and we're talking about bugs in the kernel here...