Because the plate armour is the symbol and single most expensive item a noble would have.
Actually, I disagree with that. The most notable symbol of wealth a noble would have is the size of his estate and the number of people who work directly for him. Outside of his lands, the most notable symbol of a noble would be the number of servants he carries around. A noble who has a nice armour but carries his own sword is not as rich as a noble who can afford to travel in silken clothes because he has two layers of bodyguards around him at all times.
In battle, sure. The armour is a battle equipment first and foremost, and only marginally a status symbol.
