In all fairness, I find your title interesting to discuss with. Here are my insights why 21 million btc can't reach out every single people in this planet. These are the reasons IMO:

1) Not everyone of us like btc or crypto in general, there are exisiting people which hate it and refuse to use it (e.g. bankers)
2) Not all nations have the required technology capable of using crypto. In short, technological gap. Best scenario is the life of rural tribes all around the world.
3) Even though we assume that world's population is only to 21 million, each one of us might still don't have a chance to get ine because of losses due to forgotten private keys etc.
4) In this world, equality is just an ideal concept. There will be always top and bottom and it all started because of greed. Many got nothing, some got ample and few got plenty. (I hope you know what I mean)