They do, they start as toddlers, they practice the language 24/7, and they have a decade to master it. Adult people simply don't have this luxury. Kids are not really a model for learning as an adult.

Learning is an effort that needs to be put. Preferably, in a time frame narrow enough you don't forget why you even started learning. And when optimizing the language learning process, grammar tables are useful.

A lot of time in school is spent learning grammar. And one (small) part of it is conjugation - you actually learn the rules. For situations where feel guides you wrong. Or for arguing - if there is disagreement you solve it y rules.