If it is possible to carry extra batteries, fully charged, there are circumstances where that is convenient. For example, when hitchhiking through Europe, and staying a week or two in a city, it is possible to get a local SIM so that people you meet have a local number they can call.
The problem is "how do you keep those extra batteries charged". Which is one of the reasons why non-replaceable batteries started taking over - first, the vast vast vast majority of users don't have extra batteries, and all will probably tell you they don't even know where to buy one right now if you needed it.
Second, unlike the early days of cellphones which came with charging docks, charging spare batteries is a serious PITA requiring manual intervention that for most people, will never happen.
E.g., you change the battery on your phone, so you have a dead battery. You then put your phone on charge at home. A couple of hours later you then have to go to your phone, swap out the batteries and charge the dead spare you have. Great if you've got the discipline to do it, but most people forget and now you have a charged battery and a dead one. And it's likely you'll forget the next day as well.
Some premium phones do have charging dock accessories so you can avoid this needless manual intervention midway through charging by putting the spare in and charging both simultaneously using a spare charger. (And often, the dock serves as a convenient holder to keeping the battery protected).
I've done it. My old phone came with 2 spare batteries - and several nasty bugs that caused it to drain them quickly. I carried the phone and its two spares with me. More often than not, the two spares were generally discharged because I kept forgetting to swap the batteries again to charge the dead ones.
I now just use an external pack, which I can charge at the same time I charge my phone - put phone on charge, put pack on charge, forget all about it. And if I forget to charge the phone (and pack), the pack holds charge to recharge my phone. But I'm far more likely to put my phone (and pack) on charge together than to remember I need to swap batteries after a few hours of charging.