Walkie talkies require a lot more power than smartphones have. FRS is 2W, ham radio handhelds are usually 5W, smartphone Wifi is 30mW. They also require a long antenna sticking out the top. Smartphones have UHF range antennas so possible to fit them but won't be as good.
What is disappointing is that LTE Direct and 5G Peer to Peer didn't go anywhere. They do LTE or 5G directly without towers. The range is 1km, not walkie talkie distance, but enough for a lot of things. There are some first responders phones with but it doesn't seem to be available for public.
> Walkie talkies require a lot more power than smartphones have.
Yes and no. "Real" handhelds could broadcast at 5W but the Walkie Talkies that Radio Shack sold[1] were in the unlicensed band and limited to 10000 uV/m which is well within capabilities of a current cell phone power wise, as you note this would not be efficient with their antenna arrangement :-)
That said, it would be fairly trivial to use websockets to create a "PTT Walkie Talkie app" (think 2 person zoom meeting with no video)
[1] Exemplar Realistic TC-500 page 56 of this 1991 catalog: https://www.radioshackcatalogs.com/flipbook/1991_radioshack_...