The US should want to keep the talent it selects and trains. These are people who build the economy, generate IP, and create jobs in the long run. Not only that, but these people who have spent 4+ years in the US university education system are extremely well-adapted to life in the US and will thrive at making the US a better place.
(Yes there's OPT, but it's not long enough and doesn't provide a path to permanent residency.)