Bogus nonsense. Sign your internal certs with an internal CA, make your clients trust the CA. Use it properly.
There's no single trust store: the OS has one, Firefox, Java (cacerts), Python (certifi), Node, Go containers, all your Docker images, ...
Failure? People just toggle TLS verification off.
Easy to do in a one-man shop, but almost impossible in any big company. Try grepping for verify=False / -k / InsecureSkipVerify in any fortune 500 and you'll find plenty
The only reason is laziness.