Because getting a frame builder to put on cantilever studs is going to cost you money + money for new paint. And still the frame will not be optimised for mud and big tyres.
Fatties fit fine? I can paint it my self for free, why not put v brakes on a steamroller, just something different and looks like a lot of fun, changing frame isn't really any cheaper or less hassle.
I do like the cross check and pacer, but yet to decide
Why would i want to do that?