[hinkley]

Not “everyone” does that. You have individual files doing their own initialization when they get loaded. Including loading other files or modules.

They might do it for testing purposes.

[bluGill]

That does happen. Still there is a reason many avoid it. Probably every significant project has places where they do that. Still if it isn't in main it is always a little "magic" and that means hard to understand how the program works. (or worse randomly doesn't work because something is used before it is initialized)