The answer basically comes down to the issue of saving on RAM vs. saving on ROM. (RAM = amount of memory need to implement the algorithm, ROM = amount of memory needed to describe the algorithm)

Video game programmers have to care about RAM, while the universe (in its capacity as a simulator) does not. That's why programmers generate only what they have to, while the universe can afford to just compute everything.

However, I asked the same question, which is what led to the blog post linked above, where I concluded that you wouldn't save memory by only doing the computations for things observers look at: first, because they check for consistency and come back to verify that the laws of physics still work, forcing you to generate the object twice.

But more importantly (as I mentioned) because the 2nd law of thermodynamics means that any time you gain information about something in the universe, you necessarily lose just as much in the process of making that observation (for a human, it takes the form of e.g. waste heat, higher-entropy decomposition of fuels). So by learning about the universe through observation, you simultaneously relieve it of having to store at least as much information (about e.g. subatomic particles).

(This argument has not been peer-reviewed, but was based on Yudkowsky's Engines of Cognition post.)