The reason this happens likely isn't negligence, but is instead a fundamental flaw in any kind of two-sided car marketplace. First, you have to buy the vehicle, and in many cases when you buy a car you have to pay off the seller's loan. In this case you don't get the title immediately - you have to wait to receive it from a financial institution. But you can't sit on a car until you actually have that title in hand - used car margins are razor-thin and cars depreciate incredibly quickly - so you need to sell the car first and hope you receive the title in time. This entire process is made even more complicated by the vast array of small banks and credit unions you have to deal with during the payoff process - many of whom have byzantine processes and can't possibly conceive of a car dealership that doesn't operate like the local auto lot.

I have no particular love for Carvana but I am sympathetic in this particular instance.