Keep in mind: 90% of people you meet IRL, day to day, are true neutral. Even people who seem extreme probably don't deviate very far from the center.

Think of all the ways a normal person interacts with the law. Yeah, we follow big laws like "don't kill people". But do you speed? Eaten a grape in the grocery store? Snuck out of work 5 minutes early on a Friday? Enjoyed 420 time? We find little ways to avoid laws all the time. We also find great ways to avoid actively doing good - sure, you add $1 to charity when you swipe your card at checkout, but is that "doing good"? When was the last time you actually went out and did active good?

TN is the baseline behavior for an average person/NPC.

"Good" requires just as much activity as "Evil". Both of them are active pursuits. Neutrality can be active neutrality, sure, but it can also just be a passive not doing anything. Keeping your head down, not getting involved, just trying to live your life and get by - that's neutrality, not good.

Likewise, "Lawfullness" is just as active as "Chaos". A Lawful character actually tries to follow all the rules, whether it's the laws of the land or his own personal code of ethics. We have names for these people in real life - Rules Lawyers. We refer to them as having a "Stick up their ass" or being "rigid". IRL, we don't like lawful people. But ... neither do most people like chaos. Someone who parks across four parking spaces. Someone who comes in half an hour late and leaves two hours early. Someone who throws a kegger at 11 pm on a Tuesday, or mows their lawn at 5 on saturday morning.

An "Active" TN will go out of his way to remain TN. He judges and balances. He weighs opinions and actions. He sees a person getting mugged in the street and contemplates what each course of action means in the greater scheme. A "Passive" TN sees a person getting mugged, pretends not to notice, and hustles by, because he can't help anyway, he doesn't want to get stabbed, and it's not really his business.

A good person tries to stop the mugger. An evil person joins them in the mugging.

TN is the center because it's the "blank", as well as being an active alignment choice. TN is the base against which you judge what is good, evil, lawful, or chaotic. Like all alignments, it is fluid, vague, and open to interpretation, but it's the "average".