Because Bitcoins are not "created/made" but instead it is "distributed" through Coinbase. The total epoch with block reward as incentive are fixed to the first 34th epoch (.2140-200
Speaking of 21 millions, there could not be "21 millions Bitcoins" either in circulation or in existence, why ?
  • The supply cap is less than that
  • OP_Return or some other scenarios can make UTXOs unspendable
  • Some Block rewards aren't fully-claimed (intentionally?)