Isn't technically everything open-source?
Sucuk@kbin.social to 
I don't know (but wanna learn) programming, but, for example, can't you inspect the code of an app if it's installed?
(yeah this is kind of a stupid question.)
EDIT: Thanks for the clarification, guys!
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You said "most of the time" - when is that not the case?
Some programs are distributed as "scripts" (in a scripting language like BASIC, BASH, Python, JavaScript, Lua, etc) which are stored on your computer in human-readable form and only converted into CPU machine code when you run the program, through an "interpreter" program.
Of course, everything boils down to binary machine code in the end, because those CPU instructions are the only language that your CPU actually works with.
There are some programs (especially on Linux) where they don't distribute compiled binaries and you just download and compile the source code yourself to be able to use the software. This can be because of legal reasons, technological reasons, or even just because a developer wants to be very transparent in what's being run on your machine.
This is especially common with software alphas (either for new software, or for testing updates to existing software) where they just don't bother compiling it for every type of system when it's really just for use by a handful of developers while they're actively working on or testing the code.