It seems that instead of adding side mirrors (which I'll agree are going to get knocked off a lot) drivers in India instead created the custom of blasting their horns when passing, to let drivers know someone was in their blind spot

 

And so they painted the words "Horn OK Please" on the backs of their trucks. 

This created so damn much noise, that the the transportation department of Maharashtra, the state containing the city of Mumbai, outlawed the phrase to diminish the noise pollution caused by the honking

https://www.atlasobscura.com/articles/the-origins-of-horn-ok-please-indias-most-ubiquitous-phrase

Why do Indians use the car horn so much?

The complete disregard for side mirrors. No one uses them. Some trucks don't even have them. Others have their mirrors pushed closer to the vehicle because to extend them outward could invite a collision with another driver. 

So with that in mind, truckers behind the wheel of India's ancient truck fleet -- at least 40 years old according to Singh -- invite drivers to honk as a means to advise them that they are about to pass.

https://www.forbes.com/sites/kenrapoza/2014/05/13/one-plausible-reason-why-indians-love-honking-horns/?sh=20a86f84272a

https://www.theclassictimes.com/transporte/223-camiones-india