It isn't just talking to yourself that I mean, your childhood family will say things you wouldn't think of. In the same way the parent comment says that adults won't correct grammar because it's impolite, adult visitors to your house won't tell you to pick up your toys, or lift your arms up to put a sweater on, or brush your teeth, or how to set the table. You won't get constant feedback on the day-to-day minutuae of life - and the constant use of sentences around the vocabulary. If you don't know what "toothpaste is" you're unlikely to stop brushing your teeth and go and look it up. If you do, and you make up sentences and say them to nobody, outside a specific classroom environment nobody will tell you if you're making mistakes.

Just imagining a situation of child immersion vs adult immersion vs adult trying to make sentences up:

Adult making sentences up: "I am put on coat. Two arm".

Adult immersion: "What is the word for coat?" "'coat'", "Thank you. I am put on coat". "nearly, we say 'putting on my coat <blah blah>" "Thanks, I am putting on my coat".

Child immersion: "Come here and put your coat on, we're going out, you will get cold", "No I don't want to gone", "don't want to go, say it", "don't want to go", "ok but we're going, so blue coat or rain coat?", "blue raincoat", "warm coat or small coat?", "warm", "good, now put down what you're holding, how will you get your arms in?",...

It's so much more everything, more words, more contexts, more sentences, more connections, more interactions, more moments of feedback, more continuous through the day, through every day, more immersive.

It's tempting to make up sentences you know, using words you know, harder to make up sentences using words you aren't sure of and sentence structures you aren't sure of. Easy to do in situations where you have some free time and remember to do it, hard to have it ongoing even when you're busy. And with scant feedback. Surely more effective than not generating anything, but equally effective? Can it possibly be?

Agreed, child immersion is more effective for the oral language. I just tried to say that diglossing one's interior monologue with the L2 is not only possible, but has proven effective IMX. Adults have to learn in a different way, but we do have two advantages: (a) money[1], and (b) literacy[2].

[1] with which to pay someone to instruct and correct us, who will often describe things deductively in grammatical terms, taking advantage of our adult L1 knowledge, instead of exclusively inductively by example. Compare https://news.ycombinator.com/item?id=24716468

When starting part-time language instruction, an adult learner is behind even a four year old. After two years, an adult learner will be dealing with more complex sentences than a six year old.

"Dinosaurs have been extinct for 60 million years, but there's substantial evidence that birds are the evolved form of dinosaurs today." is a fine example of a sentence that an adult learner should be able to grok before two years, even if they might parse it at first as "Dinosaurs splodge extinct for 60 million..."

[2] Unless one is seriously hipster or hermit, we are surrounded by the written word. (My italian is nearly nonexistent, but I do try to follow cooking directions in it, before falling back to the languages I have.)

One language teacher of mine observed that she could tell within a few weeks which of her students read or not: the non-readers were advanced in oral comprehension and generation over a limited conversational domain, while the readers were less fluent orally but used a much wider range of grammar and vocabulary.

Having seen celebrity twitter, I concur with her reader/non-reader dichotomy.