They thread into nutplates with a locking feature so they are retained using a method that is the same as used on many other critical fasteners, so no need was deemed necessary when it was designed.
Many thousands of hours of operation with zero reports of anyone finding one loose has now established that that is sufficient.
I suppose you could drill the head of the bolt and get some safety wire through it and anchored somewhere but access is rather restricted so it wont be easy.