One thing people don't know is that the phone network is actually a bunch of duct-taped technology that is pretty old. There has been advancements, and if you're in the US, you'll be happy to know that mobile carriers require stir/shaken handshaking, which is _mostly_ equivalent to https on the web (this is a gross simplification).
The short/simple answer is carriers don't care, because they make money when a call is placed on their network. There is also a difference between what is a valid number (digits are correct) vs a real number (someone owns a number). It is cheap for a carrier to check validity, but not "realness" - to check a real number, a carrier may have to do some sort of data request to any number of carriers to determine if the number is owned.
The short/simple answer is carriers don't care, because they make money when a call is placed on their network. There is also a difference between what is a valid number (digits are correct) vs a real number (someone owns a number). It is cheap for a carrier to check validity, but not "realness" - to check a real number, a carrier may have to do some sort of data request to any number of carriers to determine if the number is owned.