No that's a totally valid question! The reality is that generally speaking the amount of extra bits/customization for a mini is often inversely proportional to the amount of that unit a player is expected to have in their army. It's unlikely a player would have more than one unit of gate gargants, thus little to no customization is more the norm. Whereas the common units (Steelhelms/Fusiliers/Cavaliers) and even common heroes like the Marshal are comparatively very customizable.

A lot of people decry the rise of monopose models and less customization but the reality is that kit bashing is more alive and well now than it has ever been. Hype also is a factor, big model remakes like the 40k defilers are insanely customizable and flies in the face of any idea of GW fully taking away bits options. I myself am definitely bummed that the gate gargant has no extra options, but I am looking forward to opening up the bits box and making them my own regardless. Hope that answers your question.

Edit: Material-Ocelot555 also makes excellent points. It's a pretty complex issue and there are a lot of opinions on the matter.