That's precisely the point. NHL first-round picks have various paths they can take-they could be ready for the NHL, play in the AHL, or opt to play professionally in their home country.
Also, drafting a first-round player does not help the team develop. For example, if you draft a forward who is a first-round pick, he is going to expect the minutes. How do you think players drafted by other teams are going to feel?
The bottom line is simple: An NHL-drafted player has many options, three of which do not include the OHL, especially a team that has not proven that it can develop talent.