You generally can, though. All the web servers, all the frameworks, all the packages. If I wanted to make a graphing calaculator in Python I wouldn't write a reverse polish calculator or do more complex string parsing on user input, I'd use numpy and sympy, with a qt gui, probably written in qml.

I see very few domain problems where the boatload majority of the work hasn't been done for you, and you just glue legos together. Network stack? Most languages have an httplib or you could get libftp or some such from a foss repository.

The only real problem is figuring out which api is easier to use, since there usually are competing choices on a lot of these drop in solutions. qt or boost? django or bottle? backbone or angular?