In one sense, the supply of Bitcoin is increasing with each block, but it is in a very different sense than with fiat currencies, cryptos, or even gold. None of these have a known and reliable expectation of the final supply cap.
Discussion
Because of this, we know that there will only ever be 21 million Bitcoin, and the block subsidy is merely ensuring a decentralized, equitable, and predictable distribution of that fixed supply over time.