Can someone please explain how it is that some players are able to rack-up millions of PVP points when there's no obvious way they can be doing it without rigging or insider assistance or just blowing thousands of dollars on the game?

Like are they creating multiple accounts of their own that they use against each other, or getting gratis game company credits to purchase unlimited units, or are they plarium sponsored game bots — or what exactly? 

Their points are so far beyond any "normal" account that it just isn't reasonable to assume they're playing by the usual rules.

If that's not the case, and it can be done by any average player with a single account ( no alts ) without spending a small fortune, can someone please explain how?

Hitting your own alts is indeed an option, but I don't think it's a very sustainable one, as it takes time to build so many troops (or money to revive them).

Why is it so weird that people pour large amounts of money in the game? I know a few who used to (and maybe still do) spend thousands of dollars a month.

I am unaware of cheap ways to do this.