Can anyone please explain to me WHY people are using CAR COOLANT in their PC's cooling system?

well

1. ethylene glycol will NEVER turn to battery acid (physically impossible)

2. The main component of car coolant IS water. At the low temps that computers run at there is NO cooling capacity difference between that, pure water, or most other common coolants. This is because the fluid never gets heat soaked, it is always the radiator capacity which is the limiting factor of the loop not the coolant used. 

3. MOST mainstream purpose built pc coolants run with ethylene glycol in it....

https://linustechtips.com/topic/1076406-the-best-d...

4. Any conductive coolant can short out a computer, only a few non-conductive purpose made coolants exist and they are VERY expensive .... so that point is moot

5. Glycol does not eat away seals in computer coolant loops... 

The reason ethylene glycol is used in almost EVERY mainstream computer coolant is as follows

1. its cheap

2. its Biocidal and Anti corrosive

3. it is very stable when not exposed to high temps (200F+)

4.fairly safe as long as you dont drink it like a brainlet

People make custom mixes so they can save some money... as it will generally be cheaper than buying the purpose built pc coolants

yes they can but your talking about a small amount yes anti freeze is good as it does 2 things , it doesnt freeze up and it doesnt corrode , but the your cooler is either copper or stainless , you just need distilude water and way . anti freeze is a bit of an overkill .

Ethylene glycol (anti-freeze) is also used in AIO coolers like the Corsair H100i and those things last several years. Your information about anti-freeze eating gaskets is also incorrect and I think you're mistaking Dyed Coolant for Anti-Freeze.

Here's a real shocker..... There are almost always rubber gaskets the Thermostat and a Car's Radiator is connected to the Engine Block with a rubber hose.

The real concern with Custom loops isn't the gaskets giving out, it's corrosion. Mixing metals like Silver and Nickel creates an electrolysis effect which causes the Nickel to erode. This causes the flaked off material to settle on the radiator and waterblock.

Wrong again, Jan. Ethylene glycol does not turn to battery acid. It is commonly used as an additive to coolant to inhibit corrosion and doesn't eat away gaskets.

they don't know -- that's why

Nice troll.  Try again.