Purchasing cars often requires haggling. Purchasing computers rarely does.

Also, cars are often bought used and it is in the interests of the salesman to conceal information about the used car from you such as hidden problems. Computers are more rarely bought used, rarely have hidden problems that can impair their long-term functioning but are not obvious, and when bought used are often not bought for a price high enough that it's even worth the seller's effort to deceive you about the status of the computer. Furthermore, computers cam be bought in pieces and cars cannot.