Because units die quickly and you can easily go recruit more?
Armor was already owned by the family, or bought with saving specifically for that purpose. Easy reason why nobles would start with armor without seeing any deductions anywhere.
But who cares what kind of armor it is? Most people have never roleplayed their armor. Then, of those I have seen, I've never been inclined to believe that most who do RP having armor RP it as being full plate.
What is the point of this feature? To make the wealthy bastards who hogg dukeships without ever doing anything invincible? As if wounding them seriously enough to make them lose their position wasn't hard enough already.
This feature adds nothing to the game, it just gives incentives to whore gold and makes stronger those who do.
Nobles do start with armour, they simply don't start with high quality full plate armour. Chain mail would be a common armour even for relatively rich knights over much of the medieval period, or something like plate mail. But besides adding some sort of armour scarcity to the game, I'm not sure I see much benefit either.
