I've always wondered - why isn't source code disclosed in a software patent? In most other mechanical inventions or even pharmaceutical ones, the type of disclo

I've always wondered - why isn't source code disclosed in a software patent? In most other mechanical inventions or even pharmaceutical ones, the type of disclosure required does include a fair bit of detail - perhaps blueprints of some sort and certainly lots of diagrams and explanations. Wouldn't the software equivalent of that be the source code, rather than, say, screenshots?