Python project, installation only works with "pip install -e .", but not "pip install ."

I've made my first python project and uploaded it on GitHub.

Upon installation with pip, I get different errors based on whether, from the root directory, I execute

pip install .

or

pip install -e .

I was able to recreate this error by having a project with these elements:

Dir tree:

.
├── my_package
│   ├── __init__.py
│   ├── main.py
│   └── test.py
└── setup.py

main.py:

import my_package.test

test.py:

print("hello there!")

setup.py

from setuptools import setup, find_packages

setup(
    name='my_package',
    version='1.0.0',
    description='Description of my package',
    packages=find_packages(
        where="my_package"
    ),    
    install_requires=[]
)

This would, after executing

pip install .

python3 my_package/main.py

give this error:

Traceback (most recent call last):
  File "./my_package/main.py", line 1, in <module>
    import my_package.test
ModuleNotFoundError: No module named 'my_package'

Said error does not appear when main is called by using

pip install -e .

python3 my_package/main.py

giving back the expected "hello there!".

I managed to fix the problem, understanding that the issue was having the main script inside of the package, while importing files from the package itself.

But I still do not understand why it would work anyway when setting up the package/app in development mode...

Question: Why does that happen?