Because very less amount of people invest in their ICO through bounty.
Reward allocation for bounty is just a small percentage of a whole token distribution. Meaning even it's profitable or not to the project, the amount can put on risks to boost advertisement and exposures at any platforms of fields.
Advertising thru bounty sometimes create a powerful hype that even bounty hunters itself will put money on the ICO. So much hype that once it reached other platforms, it will result for much more impact.
And if that really won't work and not effective, we shouldn't see so much bounty programs today.