The reason it's not in Data.List is because there are a bazillion different splits one might want (when I was pondering the issue before Brent released it, I had collected something like 8 different proposed splits), so no agreement could ever be reached.
It is curious though that the Python community managed to agree on a single implementation and include that in the standard library… So it is possible :)
This is sometimes cited as the advantage of a benevolent dictator-for-life. I remember there was lots of argument when 'join' was added as a string method (vs. should it be a list method). In the end, Guido decided on one and that's what went in. Fortunately that particular dilemma is one forced by single-dispatch OO and doesn't apply to haskell :) I also wrote simple 'split' and 'join' functions that behave like the python ones. I use them all the time. It doesn't bother me that there are lots of other possible implementations, the simple 'join :: String -> [String] -> String' and 'split :: String -> String -> [String]' versions work in enough cases.