I do not understand why it is a habit of many people to toot the car horn when leaving a place they have visited socially.

Back in the mid-eighties many cars had indicator stalk mounted horn buttons and it was easily possible to make a very short dit-dit noise. Those with a center button were often a simple button. Much more control. Also, most cars had a single (and weak) horn. Not so bad.

Current vehicles usually have two horn devices set up to create a slightly discordant sound. This is purposely designed to get more attention and is therefore more useful as a warning device.

Also, horn activation lacks fine control with the horn button mounted under the layered mush of the steering center - it is designed to prevent accidental activation and therefore requires a lot of force and therefore less control.

And then there is the noise insulated environment of current vehicles.

The end result is usually two blasts.