It is a holdover from an old tradition. Back in the day, a far more deadly day, sometimes the matcha was poisoned. The gold on the inside with the tea would show the contamination by tarnishing. So if the gold was pristine, you were safe.
Not to mention.... the "richness" aspect of gold.
Plus the soft foil layer helped a bit to "seal" the top of the chaire.
best,
..................john