Because very less amount of people invest in their ICO through bounty.
Those people and companies that host ICO and ITO bounties are simply "printing their own money" and giving it away for simple tasks such as creating the buzz about their project. To be honest this has turned to be an effective and CHEAP way to advertiese their project and to get fake followers and group memebers so people who are interested feel it is the right project. The only downside is that they might sometimes pay bounty managers in bitcoin or any other crypto but not in their tokens.