I agree with you that there are too many coins and it is silly to have a coin for every single thing... we are getting to the point where we need to have a coin for our burger, a coin for our fries and a different coin for our drink.

The reason that they needed to create their own currency is because they needed to be able to adjust the block rewards because the system treats viewers like miners and content creators like miners, something that can't be done using an existing coin.

You can move to about 13 minutes in to learn about the new block rewards coming in 2019.

With BAT, yes they could have used an existing coin with a high supply like Xcash, BCN, Burst Coin, Ripple or anything like that... who knows what will survive and what will not.

Thank you so much for checking out the video and remember, in blockchain we trust!!