Why are teenage roles frequently played by adults, despite the fact that child actors play children?
Child labour laws.
Because of these laws, using kids in a film, particularly a television series, is a poor financial venture. They can only work half-days, they are not allowed to work overnight (which is something most sets have to do to get that bloody scene over with so we can all move on to the next scene), they must complete a certain amount of schooling, there must be a certified teacher/tutor on set that the production must pay for, there must be a certified social worker on set that the production must pay for, and so on…
Essentially, these restrictions make having minors star in a show incredibly expensive. If animatronics/CGI could be used to accurately substitute young performers with adult actors, producers and production firms would do so in a heartbeat. Currently, a large percentage of a Live Action child-star film’s budget is dedicated to the delays and additional costs that come with working with children.