Lots of reasons... 'the firm' isn't its own thing, it is controlled by the executive team, and the people making decisions about how to respond to allegations like these are the peers of the person being accused... they are likely close and know each other well, so they are more likely to believe and want to protect their friend.
In addition, often times the other executives also behave in the same way as the accused exec and want to protect themselves.
It may also have nothing to do with friendships with the accused offender whatsoever. Maybe they also hate the accused offender.
One thing big orgs do a LOT LOT LOT of is cover-ups. I would imagine (gut estimate) 1% or less of such saucy dramatic events ever float out into the public due to limited media coverage and cover ups. It's not about covering whatever accused who is accused of doing this, no, it's about protecting the fine name and reputation of the firm.
So they aren't saying necessarily "we don't care" in a typical case, more like "we care.. buuuut we care way more about our own org, so we're hushing this all up like it never happened and it will never see the light of day".
This is particularly true of very big or very prestigious orgs, your McKinseys, your Goldman, Your Harvard, your Stanford, your federal government, and so on
In addition, often times the other executives also behave in the same way as the accused exec and want to protect themselves.