I'm surprised that phone manufacturers haven't already implemented a mesh network. I guess you could kind of call Apple's Find My network one, but if you want to smuggle arbitary data the bandwidth is very low. Maybe Apple's new mobile Wi-Fi chip is a precursor to an actual Internet mesh network.
That is by design to keep you engaged. My current phone has 7050 mah and 80w charging. I charge it twice per week or so. If you have a slightly chaotic life you have to consider charging 30 times per day, hoard cables and charge powerbanks only to have it die anyway. Now I have one cable that doesn't move and doesn't break.
I don't think they themselves need to implement it. During the Hong Kong protests in 2019 they used apps like Bridgefy.
Blackberry had one, but it didn't seem to be a feature consumers particularly cared about when they fled to iPhones.
Battery life is a big deal and anything with decent speed would hurt that a lot. People won’t want to sacrifice their battery to get strangers online.