There are genetic factors, probably epigenetic ones, and environmental too.

Mothers are indeed capable of transmitting antibodies through milk, but a significant portion of it does come through exposure and vaccines. As kids start school or daycare they reshuffle all the germs they live with, as they meet new friends, go to camp, start a new school, shuffle again. Often it just feels like a runny nose or a mild fever, a little bit here, a little bit there. The problem is that the body doesn’t not exterminate a virus from its ecology. It may be kept at bay, and from critical mass, but might be extant on the skin ( hands especially ) or in the lungs for instance, just not at infection levels.

Experiencing that petri dish as an adult can be overwhelming. For the same reason that kids have a staggered plan of shots that lasts for a dozen years, you’d have to trickle in the resistance more or less that way that you came about it, or you’re not really “not overloading their immune system”

> Why can this not be addressed with vaccines?

It takes a lot of time to trust strange people enough to let them pierce your body to inject random substances. After about a century of vaccination among the "civilized", with proven results, there's still a lot of people that are not OK with it.

Newborns inherit antibodies from their mothers iirc.

Besides how hard it is to get everyone on board with vaccines even when they have been raised in a culture that's seen wild success from them for generations