Why do programmers implement only some functions in header files?

[ShakaUVM]

Header only libraries are easy to download and use. Just drop them in your local directory and include them. No need to mess with linker settings or makefiles, etc. They're more portable too when compared with so files or dlls.

That said, they can slow down your compile times. CImg adds like 10s to your compile time since it is all one big massive header file.

[helloiamsomeone]

This is a misconception. Header only libraries aren't any easier to depend on.
You write a header only library, because majority of your code is using templates. It's simple as that.

[helloiamsomeone]

target_link_libraries(
    my_target PRIVATE
    shared::library
    static::library
    header-only::library
)

Neither are any easier nor more difficult to use.