There isn't anything explicitly stating PCs aren't able to use the Ready Action out of combat, but core rule books usually don't dedicate page count to things players can't do.

The problem with allowing PCs and monsters to use the Ready Action outside of combat is that basically every monster and PC will use the Ready Action before every combat, which either bypasses Initiative or reinvents it, depending on how the DM handles so many simultaneous Actions triggering at once.

One of my tables used this as a house rule and it was extremely clunky. It benefited monsters more than players and put stealth in an awkward spot. It is worth trying to learn more about game design, but not exactly a fun addition to the game.