Because it is far, far too expensive to keep doing it.
This is my tally for tonights purchases:

One large tube of Titanium White
One large tube of Ultramarine Blue
One small tube of Cadmium Red Medium
One small tube of Mars Black.
One 16"x20" canvas.

My total?

$66.53

WHAT THE FUCK I JUST PUT 100 BUCKS IN TODAY.