[jihoon796]

Quantopian provides data, a community, and capital. All of these are expensive costs for any individual trader.

In addition, a lot of strategies don't really work that well unless you have a 'critical mass' of capital with which to capture any meaningful profit.

Execution is also important. It's one thing to have an algorithm that works in theory and is back-tested. Execution is a different ball-game because you have to deal with factors like trading costs/brokerage fees, accounting, etc.