My friend has a severe nut allergy and brings multiple epipens aboard flights. This is apparently the safest thing to ensure the proper treatment of a potential allergic reaction, especially in the case a plane cannot land immediately. Should airlines not carry epipens on them? This seems like a worthwhile investment to protect them from liabilities at the least.
Medical kits in the US contain doses of Epinephrine, though they aren't required to be in autoinjectors as far as I know (one Canadian airline stopped flying Epipens but continues to bring the drug in vial form). The airlines can exceed that standard in their kits.
There'd still be plenty of reason for your friend to carry their own, as it would be immediately accessible and wouldn't require relying on airline staff and/or someone volunteering to provide medical assistance to provide one.
Carry your own, the process of opening the doctor medical kit onboard airplanes involves showing a proper medical doctor ID to the crew.
There are flights where none of the passengers is a doctor. Do not rely on luck.