Because that is what they agreed to before the Orioles picked them. What if I agreed to do a job for $50K a year during the interview process and when I showed up to do the final paperwork to get me hired, I demanded $75K? Would the employer want to hire me? I doubt it. Even if they did, it would leave a sour taste in their mouth that would likely impact my employment at some point. This is very similar. As was said earlier, they are senior signs whose talent merits being drafted much later. The Orioles draft them earlier so they can use the difference between the slot money and the bonus money to go over slot with other draftees.