i played a custom map made by a friend of mine for 4 players, had the same thing, lots of planets, but only 1 pirate base just outside of the star for each arm (i.e. each player had 30 or so planets and had to go through a pirate base to get to the star, then another pirate base to get to another arm.
anyway, it was me against 2 unfair researchers, they got through the pirate bases before me (although i did sorta leave it on purpose...)
they both had very fortified planets, very large fleets, and it was only after my 4 starbases and my 2 super fleets kept wiping the floor with them, that they started to throw single cap ships and bits and pieces of fleets against me
i think your problem was that you started on Easy. i mean, if you had 30 pirate bases in a row, that would give even an unfair AI pause. i think empire building is only an AI priority at the beginning of the game, and late game they concentrate more on fighting (however badly) than on buidling an empire.
try it again, maybe fewer pirate worlds, and start them on unfair researcher or aggressor, and see what happens
