Just because they do it, doesn't mean that it is safe or healthy.
That exactly is the problem. Because everything, including bones, is still soft and flexible, bones can deform/contort. The ballet school I attended in France didn't teach the children the 1st position until they were 5 or 6 because children's bones are still so soft before that so that it's easy for the children to go past their natural ability which will then have health consequences (hip deformation).
And yes, ballet dancers stretch as well and they may start dancing at the age of 3 or 4 but many don't attend a professional school until school age when it's already safer to stretch.
I think the many surgeries Plushenko himself has had are proof of the danger that comes with flexibility.
Here's a blog entry from a physical therapist outlining the health risks of overstretching of young children. It's not the only one out there but I don't think it's necessary to post more than one link when they're all saying similar things.
https://www.theballetblog.com/portfolio/is-over-stretching-bad/
It's not about stretching in general. It's about stretching responsibly vs overstretching too early. There is no need for a four year old to do splits yet.