the idea is to make it limited, so you pick a number, maybe at random or maybe you pick something that has a meaning, but the point is to make it limited.
It can be changed but it needs to be supported by community and because the community is decentralized, they will not support any proposal to increase total supply. With current Bitcoin holders, increase total supply will harm their investment and for everyone, this proposal is not good at all. Increase now, increase more in future, no cap, Bitcoin will no longer a favorite investment asset in cryptocurrency market.
Because currently there is unlimited amount of money in the world, you may think that there is finite amount, but USA could print a trillion more tomorrow, it's unlimited, there is no stopping, so the idea was to make bitcoin unlike that.
They don't need any citizen consensus to print more money. Bitcoin total supply can not be increased with community consensus that I believe will never appear because it is not good for everyone.