About 20% of young and 40% of adult people in the US are considered obese [1], which has been closely associated with more severe COVID-19 cases [2].
Additionally, even young and completely healthy people have parents and grandparents. While they themselves can be completely fine after a run-in with COVID-19, their family may not be... just look at your workplace and sick leaves. It's usually all the parents that call in sick at the same time because some wave of shit is sweeping through the schools, be it the regular common cold, RSV, flu, norovirus, whatever. The problem is, us humans have adapted somewhat to these viruses, but not against SARS-CoV-2... and now it spreads to the affected generations via the children.
Can you imagine just how horrible a 10 year old feels when he learns that the COVID infection he got at school killed off his grandparents?
[1] https://www.hsph.harvard.edu/news/hsph-in-the-news/obesity-a...
[2] https://www.science.org/content/article/why-covid-19-more-de...