Having worked in the industry at that time, there were 2 main reasons they did it like that
- batteries were quite unreliable and failed often
- mfgrs couldn't afford to have one year warranties and send out field replacement units for a battery
And the reasons they stopped doing it..
- batteries got better
- battery contact failure was higher than battery failure.
- replaceable batteries compromise waterproofing
I think they should still be replacible, but they should have better connectors that are sealed off from the rest of the device. It costs a tiny bit more to do that engineering though.