Because it may be too loose, so you keep twisting it to make it fit for your body shape❤

No, they are on the inside and secured since there are several Staples that make it durable. I've done it and never had anything go wrong. I've even done it on clothes that ripped at the seam by stapling them on the inside until I can fix them when I have time. If you have a good stapler they won't come undone. You would have to manually take the pillow cover off, flip it inside out and use a staple remover. Heavy-duty staplers are used on upholstery fabric. You can make a pillow cover with your basic stapler. As long as you staple the seam neatly without the Staples being crooked or not stapled all the way, it won't happen. If you staple with no gaps in between each staple, nothing will go wrong since the case is stapled inside out. If anything were to come loose, take the pillow cover off, flip inside out and add a new staple. But I guarantee they are not going to budge if done correctly. I did this on the seams of one of my dresses where the seam came apart a little bit under my armpit, I was in a hurry, grabbed my stapler and stapled the theme. It stayed together, it didn't scratch my skin. My intention was to sew it after wearing it that one time but I ended up wearing that dress many times after that never having to remove the Staples. Make sure you staple with the fabric aligned perfectly, make sure the Staples are closed all the way once stapled and you're good to go. Any kind of thin fabric that the Staples can go through whether it's the kind of material used in the video or even clothing.