If I knew the answer to that question I'd be famous for cracking an age old mystery
I do believe I know why so many turn signal switches need replaced due to split innards.
The steering gets loose enough with age and the steering wheel has a lot of slop, requiring constant correction. When working the wheel to keep it straight while using the turn signals they keep clicking off, so the driver holds the lever in position causing the innards to split. It's a common problem