Platinum is rarer than gold so why is gold more expensive ?

Actually, platinum is more abundant than gold. It is the production of platinum that is less than the production of gold.

Because people prefer to hoard gold. It's mostly obsolete, put countries like Switzerland still prefer it over going digital.

Platinum prices are heavily influenced by the automotive industry, where it is used in catalytic converters and other areas.  With the rise of electric vehicles and suppressed demand for vehicles (especially diesels) during the global pandemic, platinum prices continue to be lower than gold (which still has a number of industrial uses).

This is one reason why many say that platinum is still the better long term investment than gold.  If the automotive market rebounds, demand will increase and prices will rise.  California is seeing the biggest spike in catalytic converter thefts in about a decade, so that could hint of growing demand.

If the automotive market doesn't rebound, then there will be less incentive for platinum to be mined and processed.  That would create greater scarcity which could cause prices to soar again as platinum is likely to remain the choice for status conscious jewelry buyers.

Gold has more uses.