It’s not fair to say that you can’t… but they don’t make it easy. Most smartphone manufacturers either glue their batteries in or hide them under layers of electronics. See the large black rectangle in the picture? That’s the battery in an iPhone. It can be changed out, if you have the hands of a surgeon and a really clean environment.
This would seem like a step back from the days when you could simply pop out a battery and pop another one in with no trouble. The thing is, people today don’t really want to replace a battery, they want a new phone. This makes all of us seem like spoiled brats, but phones really do get better every year and it seems like having an older one just isn’t worth it especially with the deals you can get from most carriers.
By sealing batteries into the phone, the manufacturers eliminate layers of plastic and contact clips that would just add unnecessary weight to the phone. Sure, you used to be able to pop the battery out, but most phones were either terminally “dumb” by today’s standards or incredibly huge and heavy (or both.) Today it’s all about phones that are thin and light, so much so that a ten-gram difference in the new iPhones is being looked at as a major problem, even though the new phones have massive improvements.
Today’s batteries don’t have outer casings or connection clips, they’re just plastic bags of goo with incredibly thin wires attached. They are ridiculously delicate and would never stand up to the rigors of average people trying to change them. And you know… we’re ok with that.