Why can't you import via import *?

Question

Python

Documentation https://docs.python.org/3/tutorial/modules.html#pa...

Now what happens when the user writes from sound.effects import *? Ideally, one would hope that this somehow goes out to the filesystem, finds which submodules are present in the package, and imports them all. This could take a long time and importing sub-modules might have unwanted side-effects that should only happen when the sub-module is explicitly imported.
The only solution is for the package author to provide an explicit index of the package. The import statement uses the following convention: if a package's __init__.py code defines a list named __all__, it is taken to be the list of module names that should be imported when from package import * is encountered.

What's not clear:

This could take a long time and importing sub-modules might have unwanted side-effects that should only happen when the sub-module is explicitly imported.

1) Why can it take a long time ? What can be in a package besides subpackages and modules? Why can't python itself read the names of the folders in the package and check for the presence of modules in them?

importing sub-modules might have unwanted side-effects that should only happen when the sub-module is explicitly imported.

2) From the fact that I myself enter the folder names in __init__.py __all__, and not the python itself identifies them according to a simple algorithm (folder = package, file .py = module), may there be any side effects? Which?