Why don't RPGs explain their game mechanics better?

Lets talk players!

I've gotten used to playing some boardgames recently and noticed how much information I felt I had compared to video games. Part of this is that boardgames necessarily need to tell you every detail of their combat system or rule set in order for the player to resolve it. Video games can hide this information behind the UI and leave the calculation hidden, but it doesn't really help the player understand what is going on.
In most cases, the tutorials provided will give you the basics on game control, but things like damage formulas, hidden statistics, and other vital game systems lie hidden. Sometimes you wonder if an ATK or DEF stat scales linearly with damage.
In fact, I usually have to go to a Wiki or Gamefaqs to even try and figure out how much damage will be dealt by a certain strike. Players blunder around with a lack of information and have to rely on outside sources to even understand the impact of the choices they make. That doesn't really sound like good gameplay to me. A game should involve us making informed decisions, and we really lack information.
Example:
When I first played Pokemon many years ago, I assumed a move like Swords Dance would be useless. The game simply said "Attack rose sharply!". Obviously this was not quantitative in any way, so I assumed that the increase was negligible and you would better spend the turn attacking. It was only years later when I bothered opening a wiki that I knew exactly how powerful this move could be. But the game never bothered quantifying anything (in fact, it doesn't explain what it uses stats for at all).
Why are RPG developers so reluctant to fully explain their systems? Not all players would definitely need this info but I definitely think we should be told how something like a damage formula would work. Not to mention it gives players a much better sense of the game mechanics.
I really think players would greatly benefit if we were given the option to read boardgame style instruction manuals about the calculations.