i’ve seen lots of warnings about computer hardware and other electronics but nothing about cellphones which one would presume are handled many times a day.
Probally because of their internal shielding and casing that isolates the sensitive harware from mild static charges.
You mean why aren’t they damaged by it rather than interfered with by it?
First of all, the case and internal packaging must be conducting enough and continuous enough on the outside and insulating enough inside that, that not much current actually flows over the circuit board or boards. Of course the signal has to get in through the antenna, but it goes through filters that limit the bandwidth that gets in and biased diodes that conduct dangerously strong signals to ground. Then the circuits of the board itself are designed to be electrically rugged.
A lot of work was done at one time on nuclear electromagnetic pulse (EMP) which can be generated by nuclear explosions, largely depending on the altitude of the explosion. Of course parts of this were classified, but some of the engineers build cell phones, especially after the end of the Cold War, and other parts were declassified or not classified. The only detail I can think of that I know from that is how to calculate the microwave leakage through a thin crack in a metal box, with adequate computer time available.
A built device won’t be as effected(if at all) by static in the same way individual components would be.
Same would go for the entire PC would not be effected by static electricity, but the stick of ram inside, if handled improperly by itself may.
Cause when you pick it up, instead of shocking you, it actually charges your battery! 