Because when you've got one big savings account it is a lot easier to look at it and say "Oh! I can afford a new car now!" and cash out your entire savings on one thing without taking the other things into account.

Having different accounts for different goals keeps a clean separation of concerns. When your car account has enough to buy a car then you can buy it and know you aren't taking away from what you have planned for your kid's college education. If you decide to take money from one account to pay for something else you have to actually think about what you are doing and are more aware of the trade offs.