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?