Price and availability.
Which is part of why "voting with your wallet" is a fallacious late capitalist trap. You don't have much of a vote and you can't use it freely.
I've lived in places not covered by Amazon and they have much better local alternatives than the Western European regions where Amazon has sucked the air out of the space. You're not going to re-enable competition in logistics by buying from local online stores, you're re-enabling competition by breaking up Amazon and nationalizing their local branches (or taxing them to hell until they can't compete, I suppose).