Meh, that really just depends on how you look at it. Personally, I think of the production wheel as being representative of government spending, rather than the total sum output of the world - there's probably a significant private sector going on, which is largely the only possible explanation for why trade routes don't seem to require you to actually, y'know, build any goods to sell.
It also just makes a lot more sense in many circumstances. How much is a credit worth? Because you don't actually get particularly large numbers of them. Sure, it's loads compared to what you need... but if you have 10 billion people all working in cash-generation buildings, then why is your planet still only producing 50 billion credits a week? That's just 2.5 trillion a year. The GDP of the United States alone for this year is forecast to be $18 trillion, from just 300 million people or so. So, either the credit is worth many hundreds of times as much as the dollar (to the point it becomes meaningless as a unit of exchange), or what you're actually seeing is just the tax revenue. That tax revenue is then either diverted into the Imperial treasury by pointing the wheel toward economic production, spent on research, or spent on manufacturing.
When you're dealing with planetary populations that are not merely larger than present-day Earth's, but often larger by several factors, the idea that any ship would take more than a week or so becomes silly. Even with just 10 population and limiting yourself to 50% manufacturing, you have 5 BILLION people engaged in your manufacturing process. That's like everyone in Eurasia going to work on 1 project. A huge-hulled spaceship should be churned out in a matter of days, provided the plans had already been drawn up (as they have been). On the other hand, if all you're directing is the discretionary tax income (i.e., everything left over after local infrastrucutral maintenance etc has been carried out), suddenly the game's relatively low values start to make sense.
From this point of view, it makes perfect sense for the player to have total control over how he points his spending.