Sure - but the player who has adopted to the college level will have an easier time adjusting to the Pro level.most rookies aren't ready. in any sport. or any job. it takes time to acclimate. not just physically or emotionally but mentally. the game is faster. it's more complex. it's new information at a minimum.
any new job has a learning curve. people don't just jump in and take over a company. you learn the ropes as you go and build from there.
If the NBA wanted 18-yos, they would take them. The NBA is trying to protect its teams from themselves. The more information they have on players, the better the judgment they can have towards hiring the player. I think if it was up to the NBA, they would set a 21-yo minimum age - to ensure that most players in the league are physically, emotionally, and mentally ready for the challenge - thereby giving the best product. That also give NBA teams a much bigger window into evaluating those players - also helping to improve the product on the court.
Again, the NBA benefits greatly from the marketing of college players before the Draft - that marketing costs the NBA nothing - but delivers household "stars" to the NBA every season. The longer the kids stay in college, the bigger the name - the better for the NBA in terms of marketing its product.