Much of the criticism of adult kids living at home comes from the "Misery Loves Company" mindset, as well as jealousy.
Frequently, adults who had to struggle want others to struggle as they have.
My cousin is an orthopedic surgeon. He lived at home while he was in medical school, so he could devote all his time to studying & he became one of the youngest surgeons. Many of his classmates hated him because they couldn't live at home & had to work part time. They made comments like, "Look at the rich kid who lives with mommy & daddy & has it easy."
My nephew is a successful commercial artist for the movie & comic industry & a professor. He also lived at home during his education. Same issue -some of his classmates who had to struggle hated him. He moved out when he could afford to buy a house at 28. A really nice house.
Many parents are the same way; they want their kids to struggle & suffer as they have.