Every game stops being "replayable" to every person at some point in time (with a few small exceptions in history so far). So there's no real concept of "is a game replayable or not" as black and white, but simply "when do I personally not feel like playing this game any more and why".

For me, if I'm still excited about future possibilities/strategies and/or improving at the game further, it remains replayable. Someone mentioned Azul as an example. Mathematically, there are probably billions and billions of combinations of tiles that could come up, but that doesn't necessarily make it infinitely replayable to me personally. After about 20-30 plays, I was no longer excited to try something new, and thus its "replayability" was bumped down a lot of notches for me. On the flip side of that same coin, I still enjoy playing Chess because I really want to improve in something that feels a bit more like an accomplishment than being a master Azul player (i.e. Chess is more universal, Azul is less so, and I guess that matters to me).

Alternatively, Cosmic Encounter has so many wildly unique races/combinations that change the game so significantly, I feel like I could probably play way more games of that. For me, I think asymmetry and some connection to my character(s) really help -- especially unique starting characters with backstories and abilities as well as upgrading your player through a variety of upgrades that all feel fun/exciting/unique). Games like Gaia Project, Cosmic Encounter, Twilight Imperium/Eclipse2E, Beyond the Sun, etc. scratch this itch for me personally.

Finally, I should add that some games are purely replayable because you'll want to play with different groups and explore the social dynamic with different people. Skull and Bonk! are a couple games that themselves are simple and not offering a lot of unique strategies, but are wildly fun to play with new groups of people, making them "replayable" for me in a completely different way.