Why wait? Because so many 17 and 18 year olds, or younger, don’t have any real idea of what their true interests really are. And those interests can change dramatically.
Many colleges have core curriculums that require courses in many different areas so that students get a taste of things other than what they “think” is their interest. And many find something totally different that piques their interest.
In addition, many if not most, college students switch majors multiple times during college.
And lastly, career choices don’t necessarily have to be aligned with a specific college major (of course things like engineering are the exception). Many students find jobs and careers that have nothing to do with their actual major in college.
College is a time for students to hone their interests and make these choices. This does not have to be done in high school.