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.