To offer worhsip is to obey the Command of Allah

It is also like asking why Hajj is observed on 9th Zul Hajjah and not on just any day convenient to a pilgrim, and why stay just one day at 'Arafah and not three days.

Obviously, if anyone stays at 'Arafah for three days instead of one day his Hajj will not be valid because he does not follow the method prescribed by Allah.

Again, a man cannot say that he will perform the Rami of the Jarnarat all together on the fourth day instead of the three days of Hajj.

Rami is the casting of pebbles on the Jamarat, the pillars marking places where Satan tried to tempt
Sayyidina Ismail علیه السلام. This also is a form of worship and a worship should be performed in its prescribed mannerotherwise it is invalid and meaningless.

Thus they raise an objection on the levy of zakah on gold and silver and not on pearl is to question the philosophy of worship.

Nevertheless, Allah has prescribed zakah on gold and silver whether in use or not, and on cash money.

source: Islahi Khutbat  Mufti Mohammad TaqiUsmani