He did this so that the Israelites would be a very strong and great nation one day without slavery. But at the same time they had to realize that they had to do their part as well. They had to learn to act by God's lead. The Hebrews were also the people who believed in God the most because they were the only people to really experience God's presence and power first hand. This meant that they were also obligated to follow God's commandments.
Someone can skip growing up and experience life very fast and become an adult at a very young age.
But by taking this short cut you could miss things. Just like the Israelites would have missed getting tested if they took the short cut through the land. Maybe if you grow up too fast you could mess up and not learn all the things you need. Very often we manage to paralyze ourselves into indecision. We analyze, we rationalize, we procrastinate.