Because it’s a pointless, clumsy and inefficient solution to a problem that doesn’t really exist.
Sure, there are bottle cage bosses on the seat tube, but only because the frame builder was daft enough to put them in a place that made it almost impossible to get even a small bottle in.
Yes, fat tyres and rims are available, and it could even be argued that they have uses for riding on sand or snow. But the downsides and overly complicated build solutions are well documented – does it really make sense to build a fat bike for UK conditions? No.
I “could” build a bike with 19 wheels that carries 6 people sitting backwards whilst playing backgammon…
Should I? No, because it would be just as pointless!