It's the rules of how they must account for the value of the gold they have. Gold is valued at the price paid. Then, it is valued at the price sold. If there is no sale for more than a century, it stays on the books at the price paid. Once a transaction happens, the numbers update. Then, the gain that everyone knows is there is 'realized'. It's like if you mined Bitcoin in the early days. Your gain is only 'realized' when you actually sell it. Until then, it is only theoretical.
Mark-to-market accounting systems are one way to deal with this quirk, but they create their own issues.
What I was trying to get at is that there are other ways to update asset valuations besides daily (market-to-market) and once (price paid) – those are just the extreme ends of a spectrum. What makes sense really depends on the asset class and how long you're holding the positions. As for "It's the rules", I'm aware that there are strict accounting rules for companies and regular banks, but do those really apply to the central bank in the exact same way? (A central bank typically operates on a much longer time scales.)
If the central bank doesn't follow rules, who would trust it? The central bank's entire purpose is to put national trust into individual banks; both assuring investments (accounts) and establishing base (prime) loan rates.
A central bank answers directly to the government, not the judiciary. But it still answers to power, and follows established rules.
I'm not saying it shouldn't follow rules, I said the rules might be different.
A balance sheet becomes pointless if some assets are valued at today's prices, while other assets are valued at their price from 100 years ago.