It is a fairly common question.
It's solved by regex already, so it's not something that engine developers normally go out of their way to specifically add in after already supporting regex.
Also, while it can be difficult for a beginner to think through it, it wouldn't be hard for an experienced developer to put it together in half an hour with the math tools provided, which you can then utilize. That's all libraries and behaviours and plugins are in the programming world: people who have put together ready to use code so you don't have to yourself.
Now that I actually look and research a bit, of course it's already been done before. stackoverflow.com/questions/1990512/add-comma-to-numbers-every-three-digits