> everybody already knows that people who buy car parts are likely trying to fix their car

But people who only have one car can probably be forced to pay more than someone who is buying a part for a second car. Someone with more money in their bank account can afford to pay higher prices. More information might not always help them in every case, but it doesn't have to as long as it can help them enough of the time. Companies expect to make massive increases in profits by introducing discriminatory pricing.

Other sellers want to maximize the amount of money they can squeeze out of their customers too so they will use whatever information they have to set prices as high as they can get away with. Some places may not jump on discriminatory pricing as quickly, but consumers don't always shop around either and the companies making money hand over fist by jacking up their prices will be able to pay for more ads and better rankings in search results.