The state space you get when using real programming languages like Python is much, much larger than the one you get when abstracting your system design into TLA+. Thus when testing real systems you can only explore very small portions of the state space. This is a real thing people do, although it isn't yet widespread - the term to look for is deterministic simulation testing. Making a DST harness that can handle exploring an application state space without requiring large modification to the application itself is very challenging. Currently Antithesis are the only ones I know who have done it (disclaimer: no connection to this company, I just think they are very cool).