Bitcoins are analogous to gold; both gold and bitcoins are limited in total amount, making them an effective way to store money without paying the "inflation tax". However, there are currently many problems with cryptocurrencies, such as the division into "pure" and "dirty" crypto, and I am not sure that cryptocurrencies are more convenient to use than gold. And this leads to a new question: why the gold is not growing as fast as bitcoins? Maybe buying gold is illegal in the US/Europe?
By "growing as fast as bitcoin" you mean the price of it? Because gold has around 10x market cap of btc, making gold more expensive already. And there's only so much room to grow.
Also there's dirty gold as well, like dirty bitcoin and dirty money. FATF regulations might want you to prove the origins and trail how you got any asset you own.
Usually when you buy gold in commodity trading exchanges, it's not physical gold. You are actually buying synthetics representing the value of gold. Sometimes they are backed by actual gold.
Also it's not illegal to buy gold, but it's illegal to buy and sell stolen gold for example, just like stolen anything.