There are big differences between values of gold and silver, or gold and cattle in the amount of nisab. For example, 30 cows cost almost 30,000 -40,000 TL whereas 85 gr. of gold almost cost 2,000 TL. Is this normal? Shouldn't new adjustments on amount of nisab be made?
Gold and silver are monetary values. They should be circulating continually. They make as much use of people as they circulate. 100 gr. of gold would be as useful as 10 kg. if it changes 100 hands in a month. Besides, people may be keeping it aside and not putting into circulation. Non-circulating money does not make use of anybody. Zakat forces people to put the money into circulation.
Cattle are not like money. They require constant maintenance and even the stingiest person on earth has to let people avail of them. The milk, butter, cheese, meat, etc. of them are sold, which lets people gain some of their needs from them. Therefore, the zakat of the cattle that cannot feed on grass more than six months of a year is not due.
You may think of this system as incentives nowadays. Agriculture and breeding are the most encouraged branches. However, saving money and keeping it under pillow is never appreciated.