The history of phone tech is "can we get more done with less joules?", while a pager is "you have one job".
Even when a pager is implemented on top of normal cellular networks like 3/4/5G, it's still better because there's nothing else on the system to drain the battery.
But it doesn't need to be on those systems at all, it can be an even less 'smart' radio receiver such as POCSAG system, on its own frequency, chosen specifically for getting though concrete etc., and disregarding any concerns about bandwidth because 1.2 kb/s is probably more than it needs.
It actually makes a lot of sense. A lot of pagers operate on lower frequencies (~100 mhz instead of 400 or 700) that can penetrate way deeper than the higher frequencies used by modern phones. Plus the data rate is substantially lower, which acts in the favor of getting reception.
So yeah it makes a ton of sense. These are very different devices operating using different frequencies and protocols.
Pagers are typically unidirectional. So a client's inability to transmit back an ACK deep inside a building is not a constraint. Just crank up the broadcast power on the network side.
