I've been perplexed as to why this hasnt been built in to the python runtime since day 1.

Python was first released in 1991.

Type inference has been around (as in "used in programming languages") since the early 1970s. In the beginning python hit a sweet spot: glue code that wasn't exactly write once, too difficult to do in bash, and too small to go all the way and use C++. However, it was not without its own design issues.

And day 1 of when type hints was introduced was in 2014.