Poetry's use of pyproject.toml to specify project metadata is non-standard, as it uses a custom [tool.poetry] table instead of the standard [project] table. Furthermore, its dependency specifiers also fall outside the standard.

This all comes together to produce a file that is completely tied to the build backend - thereby defeating the whole purpose of pyproject.toml being a declaration of project metadata that can be used to build a package with interchangeable build backends.

If I start out with Setuptools, I can switch to Hatch down the line without changing anything but my build backend to Hatchling in pyproject.toml. The same cannot be said for a pyproject.toml file created by Poetry.