Because the stereo doesn't spy on us (hopefully). If it did, I wouldn't buy one, as it's not a necessity, either.
The zipper also doesn't spy on us... yet? When smart zippers become the norm and you can't find jeans with dumb zippers, I'll return to using buttons even if they're a bit annoying to deal with.
Good luck finding a modern car that doesn't have a stereo. And continuing the analogy, good luck finding jeans without a zipper. When the only affordable and available options spy on you, it's simple enough to keep them air gapped from the internet... Electing not to own these devices at all is a much tougher sell.
The zipper also doesn't spy on us... yet? When smart zippers become the norm and you can't find jeans with dumb zippers, I'll return to using buttons even if they're a bit annoying to deal with.