Remember that Ethereum's merge process started a few years after launching its network, whereas Bitcoin started in 2009.

Thus, Bitcoin's blockchain has reached the point where it would be too diabolical for it to successfully make a PoW --> PoS transition.

We have reached a point where it would be better to just focus on reducing CO2 emissions associated with the mining process and successfully managing the e-waste associated with ASIC miners than to go through the entire rigmarole of PoW to PoS.