Simple answer: Writing a responsive site is much faster and easier to maintain than creating two versions of the same site. And it wouldn't it be just two, you also have to support tablets, various desktop sizes, and various mobile sizes. Custom versions for each one isn't feasible.

Also, if you care about page and weight and optimization, your site will be light everywhere, and the criticism of shoehorning a big bloated desktop site into a phone won't apply. This is not that difficult to achieve.