Seniors in high school are not mature enough to decide what they want to do for the rest of their lives. Choosing a career is a big deal and is not something to take lightly. You want to choose something that you will look forward to doing everyday of your life and figuring that out takes time. Seniors should not be expected to make such a big decision at such a young age.
After learning for so many years, high students have already built up their thoughts and approached different fields. They clearly know what they like, what they are good at. It's a effective way for them to reflect on their previous learning process and understand themselves in a deeper sense. High school students shouldn't stick in a rut, like the elementary and junior stage to passively absorb all the knowledge. They should pick up the knowledge selectively and have a life goal in their mind. They are mentally developed actually. Hence I think high school students have basic knowledge in their mind and it's a good time for them to consider their future in the long run.
I think there are a great many mature, intelligent young adults out there who realize their calling at an early age. What else do you propose these young people do? Work part-time at McDonald's, live in Mom's basement, and play Call of Duty until they are 30 years old? At the least, get the pre-reqs out of the way for starters!