The pro argument is "When do you want to start learning how to play pro ball, now, or later when you are older and have an expiration date of 30 years of age?", or "Which is better $100,000 as a junior, or $10,000 in a year? You really think your senior year is worth $90,000, when we will pay your last year's tuition if for some strange reason you flop and are not the 10 year major leaguer we think you are?"
That's why they all sign.
I went back 15 years and could not find a junior that did not sign. Danny Farquhar (doing well) was a 10th rounder in 2008, but haven't seen another one that low make it to MLB long enough to lose their luggage. It is usually the wrong move, unless you are top 3 or 4 rounds, but what do I know......
Plus every kid dreams of being a pro ball player. I guess it is hard to turn down when you lay it all out there.