"Ruby, Python & co are well integrated into many third party IDEs" perhaps you're not familiar with the level of IDE integration we're talking about here.
Most (if not all) IDE's Ruby and Python integration is BS.
We're talking about real AST-based highlighting and suggestions, auto fixes, autocomplete for all APIs available (AND your own custom modules), integration with the debugger and the build system, and in Swift's case also integration with the REPL, LighTable-style live-variables and Brett-Victor-inspired live coding environment.
