Why don't cars beep when overheating?

You can add -on anything you want to your own car.
Warning alarms are available.
Most studies prove customers are annoyed or bothered by safety systems and many say the don't need to be told.
They can read an owners guide and a tech manual so they do not like the added cost of the offence of the systems they perceive as IDIOT lights or, buzzers.
Then they ask why they did not receive warning before their
unmaintained, jalopy bursts a hose or whatever.
So the reason is reducing cost.
You can spend more on yours anytime it strikes your fancy.

Since tools can be stolen or bought by the mentally ill as well as the just plain demon posessed, I would recommend knowing and checking your vehicle often for safety.
The factory spends alot of time and money to protect against tampering for a good reason.

Drivers who can't be bothered with checking under the hood once a month will ignore beeps, bells and lights. They're always asking "my check engine light is on, so what should I do? I need to drive my car 200 miles tomorrow and it's been like this for a week.". If you don't learn the first time your car blows a head gasket and it costs you $1500 to get it fixed, no beeper is going to do you any good. Now, if it shut the radio off, that might get some attention.

a big red light comes on the dash. what do you want , a boxing glove ?

because it would alarm other drivers for no reason

I've had this happen to me a few times. cruising down the highway and then WOWZERS MY ENGINES GONNA BLOW!!! (two different cars)

I wish my cars would scream and holler at me when the engine is on HOT or say when the OIL LIGHT IS ON.

Do new cars do such things?