First off, you have a plastic (polyethylene) that is comparatively cheaper to work with on manufacturing but is much harder to deal with from the modelling end compared to HIPS. Plastic cement doesn't work on PE, and I'm not sure there's any glue that "works" well on it that can be easily worked with. I have not looked into the possibilities myself, but I wouldn't be surprised to learn that getting PE to the same level of detail and clarity the market expects from HIPS would raise the price of manufacturing substantially. That the models are designed to be inexpensive as a priority, with little else given a look in, makes it pretty cheap to sculpt and tool them; there's no concern that the rifle is out of scale or that the uniform is cut wrong or the proportions are out of whack. Not having to adhere to fine details and thus not having to pay high-end sculptors and engineers to make them work makes green army men easier and cheaper to manufacture.

Second you have the scale of sales. Most wargaming products are sold in specialist stores, and it's difficult to convince most retailers to sell a full product line; few SKUs and less shelf space required are key factors for most retailers in any market after all. Army men have few SKUs from the same manufacturer and can be thrown nearly anywhere in the store so they're sold in thousands of stores. Even if every army man retailer only buys 3 units to sell, they probably still massively outstrip any wargaming product volume just because of the sheer number of retailers. Economies of scale thus drive that price lower. Wargaming in contrast can reach hundreds of SKUs in a mature game (GW, WMH, etc) and chew up an entire store aisle. It's hard for anything other than specialist stores to justify that retail space, so the mass-market stores typically don't even attempt it. Smaller markets equals higher cost.

Competition is a similar factor there. Green army men are not really copyright-able in a meaningful way, thus many manufacturers can compete which will continually drive ways of making the product cheaper for a competitive edge. However, if you want a Space Marine or an Enforcer or an Eisenkern Stormtrooper, you have to go to those respective companies. Similar to if you just want a touch-capable phone you have a huge variety of options in the market and price spectrum. As soon as you decide you want a Samsung Galaxy or an iPhone, your options and pricing bands become much more limited. You can see competition at work in wargaming with historicals where a similar "you can't copyright facts" environment exists. The need to stay even with the competition works to the consumers favor here while a lack of competition for a given specific good...doesn't. Pretty basic economics there.

Finally you have the market realities of wargaming: Not every unit in every army is a Space Marine. What I mean is that there are some units and factions that the designers and fans really want to include because they make the game interesting/cool/fun, but really are just never going to justify their manufacturing costs on their own for years, if ever. However, basing it solely on how many they sell means the price for those specialists would be astronomical compared to the general-use troops so nobody would buy them and it becomes a Catch-22 still. Not making them at all decreases unit and faction variety, which is less interesting for the players of your game which makes your sales suffer as people wander away. The answer generally chosen in the marketplace is to not make each kit in the range stand on its own, but rather to price across the entire range so that it is profitable; in other words make it so that the more popular kits essentially subsidize the less-popular ones. Eventually, some of your specialists/less-popular kits may begin to turn a profit because they're offered in a desirable medium, and that now increases the options elsewhere. Green army men don't have to prop up the sales of an alternate line.

The concept of "units" and "rules" are things that the designers of green army men are not beholden to, and thus don't factor into their equation at all. The rules are whatever you come up with, after all. It doesn't matter if their baggie has 5 bazookas in it, because there is no play-difference for the end-consumer. If you bought an Enforcer sprue that was all missile launchers though, that sprue may not be usable in WarPath, or may be less-usable than another sprue due to the rules. That's an artificial construct outside of the kit, but it influences the design of the models which in turn adds to the cost.

Taken all together, those factors combine to give us close to the price difference you see on the shelf. There's a few more minor factors at work there as well, likely including "because we can," just not to the degree that people suspect.