This is true, but now you're explicitly depending on all of your transitive dependencies, which makes updating the project a lot harder. For example, if a dependency stops pulling in a transitive dependency past a certain version, you'll need to either recreate the constraints file by reinstalling everything, or manually remove the dependencies you don't need any more.
Also pip freeze does not emit a constraints file, it emits (mostly) a requirements file. This distinction is rarely important, but when it is, it can cause a lot of problems with this workflow. For example, a constraints file cannot include any information about which extras are installed, which pip freeze does by default. It also can't contain local or file dependencies, so if you have multiple projects that you're developing together it simply won't work. You also can't have installed the current project in editable mode if you want the simple "pip freeze" workflow to work correctly (although in practice that's not so difficult to work around).
Pip-tools does work a bit better, although the last time I used it, it considered the dependency chains for production and for development in isolation, which meant it would install different versions of some packages in production than in development (which was one of the big problems I was trying to solve).
From my experience trying basically every single option in the packaging ecosystem, there aren't really any solutions here. Even Poetry, which is pretty much best-in-class for actually managing dependencies, struggles with workspace-like installations and more complicated build scripts. Which is why I think pretty much every project seems to have its own, subtly unique build/dependency system.
Compare and contrast this with, say, NPM or Cargo, which in 95% of cases just do exactly what you need them to do, correctly, safely, and without having to think about it at all.
