Hi nodemaker! I think the problem lies in the fact that you're shooting for sort of casual, young American dialect, but haven't fully grokked all the quirks of it (yet). So it ends up seeming a little like a simulation thereof.
Also, I think punctuation and grammar are ironically even more important in informal writing. With writing that emulates speech, readers are hearing a "voice" read to them in their head, and mistakes are like breaking character (unless they're deliberate, but that requires knowing the rules backwards and forwards to begin with). Compared to formal writing, the voice is not as important as the content, and so mistakes are easily glossed over. Also, commas and ellipses and such are really important (IMO) for emulating the pauses and inflections of normal speech, and so when they're missing, it sort of seems like an uncanny valley that's not quite right.
Also, I think punctuation and grammar are ironically even more important in informal writing. With writing that emulates speech, readers are hearing a "voice" read to them in their head, and mistakes are like breaking character (unless they're deliberate, but that requires knowing the rules backwards and forwards to begin with). Compared to formal writing, the voice is not as important as the content, and so mistakes are easily glossed over. Also, commas and ellipses and such are really important (IMO) for emulating the pauses and inflections of normal speech, and so when they're missing, it sort of seems like an uncanny valley that's not quite right.