Maybe they are not used to not being able to enforce their will? People with the mental capability comply and sue later.
I can imagine that once officers have started to apply force, they're caught in continuing it - because if they have used force, they cannot just go back one step of escalation and say "Yeah we tried beating this person, it wasn't effective and not a good idea, so we did more talking". The obvious follow up question of any kind of oversight would be "Why did you use force in the first place?".
They will have to justify their use of force no matter the outcome but it will be harder if it was in error. If it is ultimately successful, I can imagine the justification is way easier.
So maybe the partial but strict oversight due to documentation and the now tolerated use of 'non-lethal' means and physical force create a perverse incentive to continue to escalate. E.g. if an officer suddenly realizes they've gone too far, the easiest way out is to look for any sign to end the situation with a reason to blame the victim, in extreme cases ending up with a victim who cannot make a statement anymore.
Not one went to the manager and said: "he's not hurting anyone - leave him be"