Packages installed using poetry are not available inside `poetry shell`

Question

Packages installed using poetry install are available in poetry run but not in poetry shell.

$ poetry add {package}
$ poetry install

# this works
$ poetry run python
> import {package}

# ...but not this
$ poetry shell
$ python
> import {package}

Am I doing something wrong or misunderstanding what poetry shell does?
I thought it would activate the virtual environment setup by poetry.
poetry --version prints Poetry (version 1.5.1)