If I'm a shop and I want to deal Fender guitars, I have to sign a dealer agreement which says that I agree to buy $x worth of apparel, parts, guitars I don't want because I don't think I can sell them etc on top of the sometimes unreasonable numbers of guitars/amps they also make you buy.
The companies have all the control and they know it. A lot of brands are also making you pay a separate fee to be able to sell online and most absolutely prohibit you as a US dealer from shipping overseas.
All of these things + operating costs make the margins at your average local music shop very very slim. I ran one of the best shops in the world and we still struggled with this kinda stuff.