At the current prices, it takes about 107 BTC for an address to become a dollar millionaire. Since this metric is tied to the dollar price of Bitcoin, it tends to fluctuate a lot. For comparison, the number of addresses that contain at least 100 BTC has a much lower volatility.
We can observe that the number of addresses with balances of over $1 million hit a record high during the 2017 bull run, while the other metric remained largely unaffected by it. This remains one of the few indicators in the Bitcoin world with a very low variance through the years.
Another possible reason is that some of these addresses are controlled by large entities like exchanges and custodians, who do not like to stray from a predetermined distribution of their Bitcoin. It should also be noted that there are over 1 million of Bitcoin which were likely mined by Bitcoin creator, Satoshi Nakamoto. These coins have not moved in over 10 years.