Because Zelensky wants to win the war. And the morale of the population is extremely important to win. This could severely hamper morale. The most unpopular thing you can do is send kids who still live with mum and dad to the war.

And “isn’t a lot to ask”, you’re not the one sending thousands of your children to die every week to defend your very existence against a terrorist state who steals your children, executes POWs, uses chemical weapons, and murders civilians for fun.

And the only reason Ukraine are “low on men” is that they’re having to compensate for a severe lack of materials by having more troops with lower quality weapons. So sending missiles and more shells is really two birds with one stone here.