Presumably a passenger came to them and asked for help extracting their phone from a crevice in a seat.

If this keeps happening, providing an on-board toolkit and cross-training flight attendants in proper seat disassembly and reassembly could well pay off.

Insurance companies and legal departments will nip that idea in the bud.

Turkish Airlines did exactly that (partially disassemble the seat) on a flight I've been on a little while ago. (Someone lost their phone in a business class seat.)

The risks seem to be lower than the ones associated with landing with a device stuck somewhere.

What I don't understand is why they don't construct the seats for less chances of things getting lost in them. Some seats have huge holes to lose things in…

Huge holes are a lower risk since objects simply pass through

Except these holes open into the seat electromechanics, where moving the seat can easily catch and crush a phone…

(Business class seats can adjust themselves with a person sitting in them, quite sure that's enough force to 'fold' a phone)

Legal departments advise management, management decides.

In this case, if the legal department paints pictures of doom, and the ops department paints pictures of cost savings, it's up in the air (pun intended) which management will pick.

A competent legal department will provide a realistic assessment of risk to go into this decision making process, not just obstruct everything.