Because companies are pretty defensive their brand and also in some cases the contracts with modeling agencies may be a factor.
Source: I work at Amazon and am working closely with product imaging at the moment.
Thanks for your insight. We're doing our best to represent the brands we link to positively. In fact, we've already had two of the brands we link to reach out to us and ask if they could be of help in any way.
Because fighting infringement trumps short term profits for larger companies.