In my Scooby Doo Lore Post the comment made about the gang ignoring Scooby’s ability to talk garnered some important attention:
Now that’s a super valid concern but it’s also canon that in the original series Shaggy is a highly talented Ventriloquist and Impressionist so it’s entirely possible that Scooby can’t actually talk at all and the whole gang knows it’s just Shaggy voicing him