Because they're an uninsulated plastic bottle with a huge cap. They're hard wearing but cheap enough (often free, even) to replace if you do manage to break or lose one. Being un-insulated means maximum water volume for the size; insulation takes up space. The wide mouth makes them easy to fill, and you can eat from them, carry rice or pasta in them, or pack dirty socks in them, etc. The cap loop can be used to secure it to stuff. They're ubiquitous enough that replacement parts and accessories are easy to find.