The biggest reason you don't have a replaceable battery is cost. All those connectors and snap designs cost several pennies per device. Add to that, manufacturers can sell more devices after the battery dies.
Every connector adds additional engineering and cost. People want cheap shit, they don't want to replace parts. The entire phone can go through a single manufacturing line and the end consumer doesn't need to install anything. Notice that many devices now have induction charging systems so there isn't even a requirement for a charging connection. It won't be too long before data cables are omitted too. Apple already removed the analog headphone jack.
I think it's more fundamental: a sealed-in battery uses less space. A removable battery needs protection on both sides of it, not bare electronics, so a battery cover, or an end-plug-in design . The battery itself needs some protection on the sides as well, not just the bare tick sack as internal ones are. The connector for it is also going to be more bulky than a minimal plug-in-once type on an internal battery.
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