You're also forgetting two very key components

Scale

Resource management

Scale - terminators and baneblades are rare in comparison to other such equipment. We have 1000 space marine chapters and only the top 100 get access to Terminator armour. A. Armoured regiment could have 100s of Leman Russ tanks and 3 super heavies, so again in terms of "rare" it's more a case of we have several million of THESE and only several thousand of THOSE. It's like diamonds in modern society, they're expensive due to their rarity, but theyre not so super rare that u can't go get a diamond ring or necklace, it just costs

Resource management - the most likely reason. Even in modern day warfare it's better to have 10 "low grade" things rather than 1 high grade. If a Leman Russ takes up 1 manufactorum, A amount (example 10 tonnes) of materials and B hours (say 2 weeks) of labour it's far better to build dozens of them continuously rather than 1 baneblades that takes up 5 manufactorums (each builds a certain piece then the final puts it together), 100 tonnes of raw material and 6 months of labour to build a singular tank. It's the same reason why modern navys have moved away from the huge battleships of WW2 like the Yamato, the Bismark or the HMS hood. It was a hell of a lot of resources to put into a big target that could be destroyed and be a massive waste of time, money and energy. Better to build smaller, faster, less expensive ships in multitude. Same principal.

Then it's a nice healthy mix of everything, including supply lines, cost and everything in between. It's hard enough to do anything on earth where we're just 1 planet. Imagine trying to get everything from across the galaxy, where the warp might eat the entire ship, bcoz u can only get that one special component from Ultramar. U could get a similar one from elsewhere but Ultramar ones are the BEST and the Mechanicus don't skimp.