Because 2015 - 2000 does not equal 20.
The project started in 2000.
It would have been a much worse language if it had been released even a few months earlier. (That was before the GLR landed.)
Also just about every time a new feature was added in the first few years the existing features were redesigned so that everything was consistent.
So despite having many features from many languages, it feels like all of those features belonged together.
Because of the time taken back then, it is actually really easy to add new features to the language now. (Try adding first-class regexes to any other existing language.)
For example, all of the normal operators are just subroutines with a special name. Which means you can create a new operator that works exactly like a built-in operator by giving it a similar special name. (From a certain viewpoint, Raku doesn't have built-in operators.)
Also regexes (which have been redesigned to be easier to read) and string literals are actually Domain Specific Languages in Raku. Which means you can also add your own similar feature if you felt so inclined.
I like to think of it this way:
A language like Python is a good toolbox.
Raku is also like a good toolbox, except it is also a fully decked out machine shop for creating new tools that no one has thought of yet.
Everyone who has worked in a machine shop knows it takes a few years to get one setup just-right.
Also there is a saying:
It can be good, fast, or cheap. Pick at most two.
Raku was created entirely by volunteers, so by definition it has to be cheap.
Which means there was only ever a choice between fast or good.
I'm glad the choice was to make it good rather than fast.
(Especially since the whole reason to create it was to make something that was more good than Perl was at the time.)