Because everyone would justly feel ripped off to buy a collector edition of a book for $100 and then have to put up with ads?. I wouldn't even buy a $20 trade if it included ads.
Maybe it's just me but I don't believe that anything I pay money for should be ad supported, if ads are included then the product should be free. As it is the only ads I view now are those served online while browsing which my eyes do a great job of skimming past, I won't watch any video content that is ad supported and I certainly won't buy any comics that include them.