Blame low activation rates. Your weapon doesn't need to "proc" in order to skill up. It just needs to hit. Taking a hit that you don't shield block isn't going to affect your shield skill at all. The same is true for Parry, which will almost universally sit at a 5% activation rate anyways - essentially taking 20x longer to cap than an offensive skill.

The best way to overcome this is by increasing the number of potential procs you can have by gathering up as many mobs as possible. As long as I kept Phalanx up, I was able to watch TV with 20 mandies whacking on my Paladin in Abyssea-Altepa just fine and got plenty of shield/parry skillups.