From one angle it's like, our food and our practices are low class and dirty until a white blonde women or trendy white guy is marketing it.
This has actually happened a lot. Cooks from my country have expressed frustration that there is no global market for their cookbooks (in English - we speak English), but when a white chef releases a cookbook about our cultural food, it's "revolutionary" (as if they discovered or invented our food) and incredibly well-received. Again, this has happened many times.
It could definitely just be a question of relatability (ie you relate more to the white person), but I also really think there are some ingrained societal beliefs about what can count as cool and trendy.
This was my understanding of appropriation. Denigrating something when its originators do it, but applauding it when someone from a more privileged/less oppressed background does it.
And it's wrong, and unfair. I agree with you.
I do think there could be other dynamics, for example, celebrity chefs who do this kind of thing already have a following/standing, so would this be the same if a completely unknown white person were to release the cookbook, if you know what I mean? I will also most often buy cookbooks/follow recipes by chefs who either live or work in England, simply because I often can't get ingredients if they're writing from another country. That's not about race, language or skin colour (e.g. I don't use American recipes, for the same reason).
I think it's quite hard to pin down exactly why it's happening in each situation, whether it's plain racism or there are other things at play. But I understand the frustration if people feel unable to use their own cultural practices for fear of being ridiculed/criticised, but then have to watch people from other cultures do it and get praise for it.
I don't believe this situation is in any way the same as a white woman buying a hair product marketed to Black women, though, as has been suggested upthread.