On old cars, turn signals blink as a mechanical switch opens and closes. I know this because in my old 1972 Ford LTD, I once took the blinker mechanism apart (well, why not?) and found a motor that spun a little disk with electrical contacts arrayed around it, like a slow distributor. All this mechanical switching fed a mechanical relay, which you could hear click inside the car when the blinkers flashed on and off.
But new cars are all electronic. Rotating motors don't run blinkers, and the electronic relays are silent. So the click you hear when you're inside the car is also electronic, which means the noise could be made to sound like anything. Today, most cars make a nondescript clicking noise of some sort, but as this PR piece tells us, that click is actually carefully engineered by PhD scientists in white lab coats, employing the latest combination of psychoacoustic principles as well as the marketing manager saying, "I don't know, I think it needs to be more 'zing' than 'ping.'"
I exaggerate. But the point is that when the car's computer is sending an audio signal to a speaker, it can send anything. So why not a snappy tune? Why not a different tune or sound for each driver? Why not a pleasing tune, for just a few bucks a month, to replace the grating sound of fingernails being clipped that ships with your car for free?
Don't get me started on the seat belt warning chime.