Why do most talented children still enter college after the age of 18?
I don't think there is a need for this. Realistically, you can perhaps push the entrance age by one to three years: that compared with the expected working life of 44 years (from ages 23 to 67) is tiny. So, in the large scheme of things, having a small fraction of people (you suggest around 5%) increasing their work life in a 4% has a negligible impact.
What are the risks? You will be putting a lot of pressure on immature kids. Some can take it well and become successful; others will drop out soon enough; and others will end up burnt and fail catastrophically. Accelerated programs require a careful assessment of the participants, and following up to make sure they are keeping up (NOTE: no idea if they actually do it). Scaling this to a 5% of the population is a difficult undertaking, and you are up for a massive increase in drop outs, among the best students, no less.
And lastly, part of the whole educational experience is growing in both knowledge and maturity. Short cutting courses can speed up the first, but not the second.
There is a much more gentler road, and it is exposing the good students to advanced materials, done through advanced classes in high school, or taking a few university level courses, as Bill Barth mentions. This allows them to learn more, but also keeps the risks low. It also helps keeping them engaged, as they can choose to take early the subjects that they like, and later in life will take the ones that also need.
Would you like your extraordinary talented kid to go to college that early? I wouldn't.
Don't rob children of their childhood. Being educated and being a brilliant scientist is valuable, but is hardly most valuable.
I feel that you can hardly skip any part of human development without consequences. A 16-years-old needs to gain experiences they can only gain if they live their life as a normal teenager. Investing only in intellectual development is a mistake; a fully competent person should have other competences as well. IQ is not everything, we also have emotional intelligence and other types of intelligence that must not be overlooked. This is especially important in case of brilliant scientists, who must be aware of societal and moral implications of their research. A person who has no broader horizons than just science is a pitiful view indeed.
Even more: allowing a young person to earn all sorts of experiences may help in the development of their overall intellectual capabilities, helping them to achieve even better results also in that particular field that is science.
Also, I can't shake off the feeling that however brilliant they might be, a kid will be much more happy if they live their life as a kid.
Last but not least, what the kid will do in their life is THEIR decision, not their parents or headhunters or whoever. Pushing their to college that early robs them of this right. The decision is made for them. Wait till their adults, and let them decide themselves if they want to go to college.
Bottom line: Because there is, plain and simply, far more in life than science. And however brilliant, the kid has the right to experience all of it.
Inertia.
In most parts of the US, going to college requires being able to do other above-the-age-of-majority things like signing leases, handling major transport, etc. There's no to little support base for underage students at most universities. In large university towns like Austin, Texas, USA, where I live, there is a program in place for advanced high school students to go to the local community college (ACC) in place of some of their high school courses to get a leg up on attending the University of Texas at Austin. This program has been in place for decades and works well. Students may still live at home and attend both their regular school and ACC while keeping their support base intact.
I attended an early entrance program that served as my last two years of high school and my first two years of university. These programs aren't common in the US, but there are several.