A more complicated design costs more money to build/test and introduces more modes of failure in the final product.
You must be newly hatched.
I have plenty of devices in my gadget box with non-replaceable batteries that pre-date Apple's iPhones, and even its iPods.
Just because something is standard fare doesn't mean it's the only option. Point is, it was quite common in the past.
Also there were no real standards for chargers. Nearly every small consumer device that used NiCd batteries came with its own bespoke chargers.
Hence most small consumer devices went with replaceable batteries because they expected nearly everyone to use non-rechargeable batteries.
Get real.