Why do the Devs when trying to rebalance or fix a "overpowered" ship or piece of equipment always seem to lead towards reducing the stats or abilities of that item, instead of raising the level of the under performing ships and systems?

People have to spend a lot of time (literally years!) and money training into one of these type of vessels and being able to fly it well. Then CCP creates a nerf and that makes that ship type obsolete. It would seem more agreeable to make the under performing ships and weapon systems more appealing by raising their stats and abilities rather than reducing the effectiveness of a popular ship type or weapon system.

Wouldn't NPC get the effect of the power balancing creep as well?

Normally I would agree with that logic. However, this game is unique in that training and character development takes years to accomplish. So when the Devs makes a "rebalance" they can effectively erase a toons usefulness and value. For example the changes announced to carrier pilots/fighters will reduce their usefulness greatly making them effectively useless expect for large scale alliance fleet battles.

too many people took the bait.