They don't. It is how science works.

There are gaps in our theories where they don't meet up to suggest that these current models are incomplete or insufficient. There is no clear signpost that ever says "you've got it all". So new models and theories are developed and new experiments are made.

Some theories we definitely don't have the experiments to test (today). So they might remain unconfirmed for a while yet.

And even if a "next step" is ever found, there is no guarantee that these next steps don't point out even more things to discover.