why is this not already a block
Aw c'mon, you know the answer to that. Our preference is to give you the tools to build the blocks you want, rather than to give you every imagineable block as primitive. And that block is super easy to write yourself, as you demonstrated.
This has, in recent years, been tempered by the need for speed. Earlier versions of Snap! didn't include FOR, MAP, KEEP, COMBINE, FOR EACH, all the options of the LENGTH block except for LENGTH itself, NUMBERS FROM, APPEND, reporter IF, multi-branch IF, PIPE, TELL, ASK, variadic versions of +, ×, etc., ≠, ≤, ≥, and I'm sure I've forgotten others. All of those can be implemented in Snap! itself.
But! We're moving back toward writing blocks in Snap! itself, sort of. In the dev version, turn on "Blocks all the way" and then you can edit all those blocks and see the Snap! implementation while still running the fast primitive versions.
I improved your code to make sure it won't go to the turtle costume:
