If you remember Nokia's batteries they were covered in a relatively thick ABS shell. They were also, compared to today, had laughably little storage. A Series 40 Nokia just did not draw that much power. The single GSM/PCS radio also sipped power.

Even if you stripped a 5G phone down to a Series 40-esque interface the 5G radios alone would use more power than a whole 3310.

In order to get the power density modern phones need they require high power Li-poly batteries. An extra 3mm worth of ABS shell is a lot of lost capacity. You can't sell user serviceable Li-poly batteries without a protective shell. You'd never get a UL rating because Li-polys are dangerous if mishandled.