My understanding is that Quantopian has no interest in being a platform for HFT. It's there to democratize trading strategies and test them in a sound way. They do a lot to have a good backtesting platform and clean the data that is available in the platform. If I wanted to automate a strategy, I'd have to figure out the Robinhood API and basically recreate what they have. Instead, I can use their platform, their backtests, their free data, and build strategies that efficiently lose money.
I think the other goal being that if you create a profitable strategy, you can enter it into competitions, trade with other people's money, and make a profit. I think the platform is a value add, I have an account and use the research notebooks. Some day when I more spare time I'll make some trading algorithms.