I hate it when people ruin the magic of a story. Especially when it's the author's fault.
It's like you're on your merry way, reading a story that has tons of great stuff (hopefully) going on and you're totally wrapped up in it when, all of a sudden, WHAM!
The author puts in an author note.
In the text.
That has nothing to do with anything.
"Btw random-reader, you'll notice my use of detail -- the abusive father is drinking gin -- to create more of an image for the reader."
"LAMO favorite-buddy! Remember when this happened to us?!?! *squeee!*"
"Love you forever homie-fantastic! Mwah!"
"OMG, can you guys not wait to see what happens next?! I'd be going sooooo crazy if I were you guys!"
"They kissed!?!?!!!! Can you believe it?!?!!?!!!"
Besides making you look like a barrel of smarts, it's annoying to the reader because whatever image or magic you managed to create (stay optimistic guys. I'm sure there was... something?) is gone.
Poof.
You just destroyed all your hard work.
Go you.
And it's not like pretending it didn't happen will fix it. You now need to reintegrate the reader back into the story and keep them interested without them being on edge for yet another random author's note to shows up (which invariably happens). Which is why in-text author's notes suck and you shouldn't use them.
That being said, there are times when using an author's note is completely fine. If you want to read some hilarious in-text examples, check out "Tales of the Truly Grotesque" by ProfOdyMalorous or "Fungus for Hire" by SJForester. Both use authors notes to their advantage by adding them subtly and with purpose(1).
End of chapter/beginning of chapter author's notes are fine too; they're not interrupting the story flow.
Basically, you worked hard to create a story, weave an image, and enchant your audience. Why destroy that with a misplaced and silly note?
(1) Mainly hilarity. See what I did there?
