Because their parents are important people, and probably made sure to scrounge up enough gold to properly equip their children before sending them out.
I personally never roleplayed as wearing full armor, nor have I seen many people roleplay such things either.
They are knights yes? No matter how important the parents the idea of every noble having full plate armour is appropriate only to late medieval period. If you go down that route, why did they not scrounge enough gold to get a better starting unit for them? Or hirer some damn healers and scouts. We can PLAINLY see how much family gold is given to a new noble, and that doesn't cover the expense of full plate armour.
My main problem with the system would be it seems arbitrary. Why pick armour and not other items a noble would need? Also the fixed cost, while expensive doesn't really end up being that prohibitive. I could easily see realms dedicate themselves in peace time to fitting out as many nobles as possible with the armour, or knights just hoarding gold for it. Would the cost be met by family gold or would it need to be gold on hand? Without some form of recurring cost the only real factor to getting top level armour is time.
