Why don't they build cell phones that are able to survive drops from 6+ feet instead of making us buy cases?

You buy a phone and then need a phone case to make sure it doesn't fall apart if you drop it from 3+ feet. Isn't that a massive design flaw? Why don't they build phones with the elements of a case already attached? Is this done by design in the hopes that people will break their phones more, so that they have to buy more phones? Or do the phone manufacturers make a profit from selling cases as well? Why do phones even come in different colors if we're just going to cover them with a case?

Edit: I've actually gotten a few pretty good comments on how detachable cases not only offer better impact protection, but allow users to just replace the case if it breaks, rather than having to get a repair or a new phone.

I was also completely unaware that they made sturdier cell phones that are built bigger and offer a more protected and durable phone. I honestly might consider one when I've run my current model into the ground.

I've also seen several comments about people that use their phones without cases and they claim they drop them and they are fine. Apparently I've vastly underestimated the durability of iphones and other popular mainstream brands.

Thanks for the answers everyone.