People go on hikes in areas where there are multiple days between charging opportunities.

Hiking often also occurs in areas with bad or no coverage causing higher battery consumption with the phone trying to connect. If you don't mitigate this (with eg. turning off or enabling airplane mode) this will burn through battery much faster than the usual city dwelling.

A very tiny percentage of the population does that. Apple is not going to make the form factor of a device that sells ~100,000,000 units in 90 days to accommodate the 0.01% of people who take multiple day hikes.