Creating an email account is cheaper and easier than that, yet Gmail and others do a great job blocking spam. Any reason why similar identifying rules can't be brought to phone numbers?

Because GMail actually blocks basically everything.

To ensure your emails actually arrive in GMail, the only sure way is using GMail yourself.

Otherwise, you’ll never be able to be sure they arrive.

That’s like every ISP only allowing people from the same ISP to call them, and then increasing the cost to creating a new phone number.

Sure, now you prevent spam calls, but you also make phones unusable.