You assume there is no early access to colleges, but that is not correct.
For decades now in the United States (at least) many colleges and universities have offered early access. Some of the various ways are described below.
High School Completion programs
“Junior admits” is becoming more common where high school students who’ve have passed sufficient number of courses in the important academic fields get admitted as a college freshman, thereby skipping their senior year in high school.
State governments may grant a High School Diploma after the student has spent several months in college.
For example, Shoreline Community College describes their High School Completion program:
Students in this program take classes and make preparations for completing a high school diploma and transferring to university at the same time. This allows students to begin their college-level studies early and go on to finish their four-year degree early too.
Some even let the student skip up to two years of high school. For example, Seattle Central College has such a High School Completion program.
Another example is the UW Academy program, for gifted High School students to attend the University of Washington.
Dual Credit programs
Some states let high school students take community college classes during the summer quarter (when their high school is on hiatus) or even parallel along with attending high school. The college course may replace some of the high school courses.
For example, the State of Washington has a few of these Dual Credit programs.
Also described in the answer by sig_se_v.
Coordinated Feeder Programs
The Matteo Ricci College is a program where a Jesuit university coordinates its curriculum with local high schools. At the end of their Junior year, successful students are admitted into the university for a continuous Humanities program, skipping senior year in high school.
College Credit for High School classes
High schoolers may earn college credit without leaving campus through either of two popular programs. Google/Bing “AP vs IB” to read many comparisons.
Many high schools offer Advanced Placement courses where students potentially earn college credits while staying within the walls of their high school building. Commonly called “AP classes”.
Similar to Advanced Placement in some ways are some programs offered by the International Baccalaureate (IB).