This is because Ruby inherits it's approach to flow control from Smalltalk, while Python comes from a C/Algol-like heritage.

In fact, Smalltalk takes this much further, such that basically all flow control (including if-then-else) is handled as message sends (e.g. if-then is just a message sent to the Boolean object taking a block as it's argument).

The downside is the syntax can feel a tad clunky. The upside is incredibly simple and consistent language grammar, while making it trivial to create new flow control mechanisms since the language has all the tools baked in (primarily first class blocks).