Language isn't just some word for word translations and a new sentence structure, it's much much deeper than that.
The only thing that is required to learn a language is tens of thousands of hours of practice.
That's not true. Most native speaks have little knowledge of the grammar and syntax beyond intuitive. They have forgotten most of what they've been taught at school, plus, they could speak quite well before going to school and being taught grammar in the first place. We acquire language by osmosis, not by studying, and even less so by studying the syntax.
The fact that they don't know the scientific names of the grammar rules doesn't mean that they don't know grammar. As somebody how's native language is very dissimilar to English, my intuitive language rules are no help speaking English. The grammar you are taught at school is descriptive of English and not a prescription. The distinction s very similar to "laws in physics". Nature doesn't really care about the rules we impose on it.
Learning grammar alone is not enough. Learning vocabulary alone is not enough. Immersing yourself to a language without a guide (like your parents guided you into language) is completely ineffective.
Grammar, vocabulary and pronunciation are equally important if you want to get fluent in both spoken and written communication in a different language.
Similarly the fact that they don't study grammar and skip the "hard parts" doesn't mean that they don't pick up/know grammar, so the original argument is moot.
If you skip over the hard parts, then you're not exactly learning a language. You might as well just learn from one of those "Hello, my name is Bob, where's the toilet ? " cartoon books.
Its a bit like those people who learn programming informally. Most of them end up writing spaghetti code for the rest of their lives.
Its the same with languages. Skip the hard bits and your attempt fluency will hit a wall at some point relatively early on. Grammar is tough, but stick at it, it will eventually become second nature. Don't be lazy.