I think there are some very convincing theoretical arguments to be made, but there is also just a very practical consideration:
Right now, a very large portion of BTC is being held in the cold wallets of popular exchange platforms. Hardcore bitcoiners will shake their heads and declare "Not your keys, not your coins!", but this apparently has not stopped traders and normal users alike from keeping their coins stored with a custodial third party. Looking at the 'bitcoin rich list', we can confirm the huge number of coins held by exchanges.
This fact would put exchange operators in an undue position of power over the network: by staking coins owned by their users, the exchange operators can obtain a large, centralized point of control over the network's consensus operations.
There are already risks present when allowing a third party custodian to manage your coins/private keys, but switching to a POS system adds an entirely new and very serious type of risk! This is a very serious risk because it is existential in nature: if an exchange operator were to abuse their control of this huge number of coins somehow (by staking maliciously in some way or another), it would affect every user of the system, not just the users of that exchange.
This problem is only amplified by the fact that you have to not only trust the exchange operators to not act maliciously, you also have to trust them to secure their system against theft and intrusion. It is bad enough when hackers steal funds, giving hackers the ability to attack the network consensus as well is, in my opinion, an untenable addition of risk.
In case you aren't convinced: This risk is not theoretical, an attack like this recently happened on an altcoin network (Steem, mid-late Feb 2020). It appears that exchanges colluded to stake the coins they held custody of, in an effort to disrupt network consensus. A quick websearch brings this article about it up.