A better question is why the companies should have the right to hide it because that is what is happening. Hardly no other industry does this.
What other industry posts exact sales numbers though? Car or phone maunfacurers only do when they hit big milestones or generally sell extremly well, but for most you won't get to hear any sales numbers at all. Outside of video games posting specific sales numbers are mostly just bragging rights, nothing else.
In general, people care about sales when they are invested in a company's future.
We are interested in the success of companies that provide us with art (like movies, TV, and games) because receiving new experiences in the future fundamentally depends on their products selling well.
If a product is driven by an individual creator (like books or music), or it's a generic commodity that's serves a function (like cars, power tools, snowmobiles), there isn't nearly the same level of interest, because new products (or functionally equivalent products) will release regardless of how the old ones perform.
For example, authors will continue to write, regardless how one of their books may or may not sell. So, we don't have a vested interest in how books perform sales-wise because we know we'll always get new books from an author we like no matter what. In another example, people don't care nearly as much about car sales because people buy a single car and own it for 10 years. If Toyota goes out of business 2 years in that's a shame, but we still have 8 years left on our current car, and we can easily buy another car from Honda which is just as good (if not better).
But there isn't really an equivalent when Intelligent Systems goes out of business, because then we'll probably never see Advance Wars or Fire Emblem games ever again...and those products are unique IPs that we get very attached to.
I think there is more of an obligation for those type of companies to release their sales data as a courtesy to their fans.