Mainly because Taim doesn't want to attack the White Tower. For all intents and purposes, it seems he is taking orders from another person and has been ordered to consolidate forces and train new dreadlords.

If he tried to attack the White Tower, they'd be at a disadvantage - despite their powers, the Aes Sedai have a much stronger defensive position than the Ashaman could deal with without substantial casualties. He'd have to remove the dreamspike to get there, and we already know that there is an Aes Sedai and Ashaman ready to escape to warn Rand and/or Egwene as soon as this happens, so he won't have as much of an element of surprise as he expects.