Most of them don't really, when it comes to OS choice the smartwatch market is incredibly diverse and so having large third-party app catalogs doesn't really work.

I personally view them more as features rather than apps anyway, as they tend to be fairly basic one-hit-wonders which will be easy to develop for Asteroid (C++/Python/QML, sailfishos like middleware), theres already a couple of sailfish fitness timer apps I was thinking of porting.

Of course the whole purpose of a smartwatch is more than telling the time, i have a normal watch for that and it has a battery life measured in years.