Buy Pearls with Vouchers

We can buy literally anything we want with vouchers: spell slots, unit slots, workers, gold, gold shields, gems, the piece of equipment in the top left corner of granny’s inventory, etc. What about pearls? Why can’t we buy pearls?

Buy gems with vouchers, Spend gems, Get pearls.

Yeah but I would rather have a direct conversion that would give more pearls for less vouchers…