Same reason we need any type of human salespeople -- sometimes people don't want to do all the research & comparisons by themselves and its nice to have someone guide you in the right direction.
Now - that being said - I don't disagree with the sentiment here. The traditional car sales model sucks and I think is ripe for disruption. But don't necessarily throw the baby out with bath water ;)
For example - I bought an espresso machine for a friend the other day. I called one of the better retailers out there and spoke to a super-helpful saleswoman who talked me through the various options, gave me differences between models, what's worth paying for and what's not, and based on my personal preferences, helped me pick the right one. It was a totally pleasant experience.
But what made it particularly excellent was that she wasn't (to my knowledge) pushing one particular brand/model. She had at her disposal about 20 different brands and guided me to the "best fit" based on my needs.
Maybe that's an opportunity for car dealers in the future -- maybe manufacturers sell directly and we get independent "car sales consultants" who don't work for a particular brand....?