Someone buys all these BTCs, through OTC or on the open market, and the fact is that these coins do not return to the exchange as was once the case in the past. While the average Joe is probably investing now to sell when the price reaches a magical $100k, big investors obviously have some other plans - but again, we know their coins are stored in custodial cold storage.
We can conclude that the liquidity of crypto exchanges is lower than before, but again we should not forget that we are currently at 18.66 million mined BTC, and even that 20% is lost (3.72 million) and 50% of what is available is saved as a long-term investment (7.47 million), we have at least as much BTC (7.47 million) left on the market. I may be wrong, but I think that no matter what, there is still a fairly large amount of BTC available at all times - at least I haven't read anywhere that anyone complained that they couldn't buy as much BTC as they wanted.