Reason mechanics use cheap coolant or pipe water (free) is so that they can charge low price and thus looks affordable. And when your coolant causes rust, head gasket burn and all other issues, you go back to them for repairs.

Either that or they are uneducated about the importance of coolant. Just do what their sifu teach them.

Either way they end up with more business which is more important to them. They don't see the big picture as usual.

You can see many people who drive cheap cars are the ones affected cause they always want cheap. So these are the cars that are always in workshops. All stem from wanting to save money on using cheap engine oils and pipe water as coolant.