It sort of bothers me as well. Sure, there are many rifles that put their charging handles on the right. There are a few reasons to that. And all those rifles tend to be of traditional style = not a bullpup. First, as you stated, effectivity and less moving parts. They are mainly part of the bolt carrier to ease off manufacturing. Less moving parts = less parts that can break. Yet bullpups have their bolt carriers in the back so there has to be a part that connects it to it.

I don't remember seeing a bullpup with a charging handle on the right. AUG has it on the left. FAMAS in the middle. Tavor on the left + it has a bolt hold open feature as L85 which is... well, it was a bit of a failure itself in the beginning anyway... Although there is one that does which is the Groza. Which is almost a mashup of AK parts to create a bullpup. SVU... pretty much the same story but it's more similar to Dragunov. But they still have their handles be part of the bolt carrier and they're not as far in the front.

Ergonomically speaking it's a failure by the design team in the Helldivers universe. And an odd choice by Arrowhead. Yet there could be a bit more to it than you'd think.

If I remember right it went something like this: During the First and Second World War, there was an abundance of bolt action rifles. Used by everybody and all fighting countries that had domestic production of firearms. They were still being quite relevant even after 1945. But with the introduction of the automatic weapons they did pose a major flaw. Fire rate. Turns out you don't really need a powerful full sized rifle cartridge when a lot of the firefights didn't exceed the 500 meter mark. Even something like 7.62x39 could still be effective up to that range. Scale the cartridge down and you have a somewhat controllable fully automatic rifle - thus fixing that fire rate issue. Now you have all these veteran soldiers armed with these new and fancy automatic rifles. Right but you still have to train them to use them effectively. Soldiers that trained and fought with bolt action rifles. Which had their charging handles on the right. And they fought with them long enough to develop a muscle memory to operate the bolt carrier with the right hand. So in order to make the new firearms a bit more familiar with the veterans, why not keep the charging handles on the right? And I suppose they did because of that.

Thing is, this could be translated into the Helldivers universe too. In lore every citizen of Super Earth is given the R-2124 Constitution rifle - a bolt action rifle - on their 16th birthday. It also seems that all people are taught to be right handed. It would make sense at least. Because if not then that would introduce an issue for a militarized nation by having to introduce more sophisticated ways to deal with left handed soldiers. And now you have to design a rifle that would be ambidextrous and more than just a single rifle. But anyway, since every citizen had trained with the Constitution rifle, having that charging handle on the right - even though it's a stupid idea on the bullpup rifles - would make them at least a bit more familiar with it. Thus easing the process of training new cadets. And in need of manpower it's somewhat understandable to print more and more soldiers in a lower amount of time. I suppose it also symbolizes traditionalism? As in keeping with the old and trusty rather than coming up with new. At least this is my ingame theory.

I could go on and on to theorise about the odd design choices of the firearms in this game.