I don't know how caught up you are with the series, but Morgan used them frequently. In episode 6.04, which is a lengthy flashback episode about Morgan and how he got his whole "All life is precious" mentality after just recently being "I GOTTA CLEAR" last time we saw him, you see that he's been using spears to fight the walkers for a while, though they were more sharpened sticks than sticks with rocks attached.
In fact, in said episode, the man he meets (Eastman) even carved the pointed end of his spear in order to turn it into a staff, which is the weapon that Morgan is currently using now in the series.
As far as the way walkers function and such, Robert Kirkman already confirmed that no, that will never be explained. And I'm happy about that. When you begin going into how they function, how it started, etc. that's when loads of plot holes begin showing up.
The BIGGEST thing that bothers me is that at the end of season 1 (1.06, I believe), Dr. Jenner stated that only the brain stem of the walker is revived, not the entire brain. Which then meant that the brain stem had to be destroyed to kill the walker. But throughout the show you see them hitting them wherever the hell on the head and killing them. Michonne has even just straight up cut the top of a walker's head off multiple times, not even hitting anywhere NEAR the brain stem, and they still die.
Whatever, though. The show has been enjoyable for me, and I didn't even get into it until around August last year.
EDIT: Also, I think that they don't use spears since most of the time they encounter walkers, they're fighting them up close, and spears suck up close.