No offense taken.
They're a bit of a long story, though.
In V1, since I couldn't do infantry models yet I was feeling rather limited in what new units I could add. There was the Zombie unit model, however, so I figured I'd add in the Mutant to have another new unit, and as a bit of an homage to things like Resident Evil and the Umbrella Corporation. It was originally designed as a cheap Infantry alternative that could regenerate, giving an option for a disposable front-line unit. Since they were just random victims mutated into monsters, it made sense they didn't actually upgrade from anything, and they were just disposable fodder.
As I got better at doing unit models, however, I started to mull upon the idea of extending the Mutant line into a line of "biotroopers" -- Future Worlds is somewhat divided up into three "branches", and the "ecological" branch was lacking in units a bit. Creating the biotrooper line helped compensate for that by giving the ecologicals more units to build. Certainly, I'm aware most players typically just upgrade existing units in the end game... but I've always been a rather poor warmonger, and thus tend not to instead only end up building units when I need them (usually when I get invaded for not having a strong enough military). So for me, I didn't mind the thought of units you had to build fresh, rather than upgrade. What's more, I added in buildings like the VR Training Centre and the Biogenesis Pods which allowed you to make more veteran/powerful units from scratch. Given that the Biogenesis Pods also accelerate the production of the "biotrooper" line and the chimeras, it did lend itself to a possible playstyle where the player can create veteran units from scratch quickly, which may work with certain civs such as Huitzil's Fairy Alliance (which encourages that playstyle).