Why does olives with the pit left always taste better than pitted ones
Just like the US dollar, olives suffer from dilution.

When an olive is picked and brined, the olive skin provides a barrier between the tasty fruit and the liquid medium.

When the fruit is pierced to remove the seed, the unprotected pulp of the fruit is in constant, direct contact with the brine liquid.

This direct contact allows the natural juices, which are protected by the olive skin in regular, unpitted, olives, to leach out into the brine liquid, reducing the flavor proportionally.

Pitted olives have their place, in stews, soups, salads etc.  For full flavor though, always go for the unpitted olives.

Have worked in many olive producing areas in the world, and the best olives are always unpitted.

You could even do an experiment to prove or disprove this:

Buy some decent unpitted olives, fairly large.  Buy a hand held cherry pitter and remove the pits of half the olives, put both the pitted and unpitted olives back in the brine and leave them on the counter for a week or so, then try them.  There will be a difference.