So then make them replaceable!
They are. YouTube video shows how to do it in two minutes.
They are replaceable. But, you also have competign demands, for things like water resistance - which is incompatible with a snap on lid. People drop their iPhones in the shower, bathtub, toilet and even outside into puddles, pools and aquariums.
So, Apple did the logical thing, they made their phones more water resistant, and they have companies out that that will replace your battery for ~$79, or will sell you the tools and provide a step-by-step video to replace the battery yourself for $~20, tools included.