There's a huge discrepancy in the amount of credits some players have.
if you make everything in the game purchasable the game becomes really difficult to balance.
"Oh this amazing item we'll charge a large amount of credits for it" But "large amount" has no meaning when some players have 10 million and others 10 billion.
Making everything purchasable means playerswith loads of credits just bypass gameplay with credits, almost in-game pay2win. It's not only a problem from a game imbalance point of view it's also bad from a gameplay point of view. ie Engineers comes out, if all materials are purchasable then folk just buy their way to all level 5 upgrades in a few days and a week later they're complaining about lack of content.
Materials puts everyone on a level playing field, you can't bypass the system, you can't just buy your way to getting all the best stuff, and "auto-completing" all the content in a few clicks.
Sorry, but RNG is worst to balance this, one may have to grind once to get a good upgrade, will get an element he's looking for at the first refresh of the mission board while another one will never get something near the level of the lucky player upgrade after a hundred tries (have you seen the range of the values for the level 5 ?), and will have to refresh the board a thousand times to get what he want, I really don't see how you can make things more unbalanced than RNG.
A reputation system based on missions becoming harder and harder to access better upgrade, that was how to do it, not the lazy implementation we have today, they should have taken 1 or 2 years more to release this gameplay, but now we have another Powerplay in game, something that can't be balanced unless you scrap it and go back to the drawing board.