Unfortunately there has to be a standard followed and this is that. In the case of the Leopard 2A6, the muzzle velocity for DM53 is displayed publicly on Rheinmetalls website. The devs do not want to start bending the rules because it creates even more opportunities that people will then expect.
If a source cant be provided, then it wont be changed.
We dont calculate penetration that way and have not for over a year: https://warthunder.com/en/news/6010-development-improved-calculation-of-armour-penetration-in-the-game-en