This limit was decided by Satoshi Nakamoto and the initial dev team at the beginning. The number is irrelevant, the important part is that is fixed and that has a predictable issuance.

In theory, the 21 million could be changed but it won’t. The network is extremely reluctant to alter the algorithm and a major change like that will never get accepted by consensus.

The value of bitcoin is backed by supply and demand, just like any other financial asset. Gold, dollars, and diamonds are exactly the same. The difference is that the dollar has an infinite supply while Btc’s is very scarce. This is why the dollar value is going to zero fast and Bitcoin trends to infinity.