It is better not to be a day trader. 90% of day trader will lose money. One reason is that the trading cost fees. The second reason you cannot sell at the top and buy at the bottom most of the time.
But yet there are enough daytraders... right.
Their incentive must be profit. I think they trade in big volumes and with that they don't need to sell at the top. When the bitcoin price is let's say 240 USD and they sell 100 bitcons at 250 USD. their profit is 1000 - 200 (fee) = around 800 usd. That's just 1 day...From the other 29 days they need just 2 more days like that and they have a nice income.
I do however think this is very risky.