Because of the levels of various types of, in this case, racial segregation common in the US, even the possibilities of people of different tribes potentially interacting in benign or more positive ways are reduced. 

It isn't that when black and white people are in one another's presence the birds sing, the sun peeks out from behind the clouds, and the universe is in harmony.  Doh.  But where there is minimal or no even potentially benign cross-racial interaction, the very possibilities for developing tolerance, at least, as a starting point, over time in what is typically an extended process, are diminished. Even the possibility of chipping away at invidious stereotypes and prejudices through relatively benign if not better interaction is reduced.  Exclusive tribal tendencies can be reinforced or even just not seriously challenged, which itself is highly toxic and as rmdr0000 notes can be fatal.

This is the cancer of high degrees of segregation in a diverse society.  If there are instances of diverse societies that have not had tensions or far worse owing in part to exclusionary tribal tendencies, none come readily to mind.  

There is a body of social science research called contact theory.  It identifies micro conditions under which cross-racial contact can lead to improved race relations rather than the opposite.  

My experiences suggest that it is almost never the case that persuasion occurs overtly and in the moment.   The persuaded person who acknowledges they have been persuaded in the moment (perhaps more so where there are males involved--there may well be gender differences in this area) can experience this as something of an acknowledgement of a kind of defeat.  Which is unfortunate but sometimes a reality.

Of course, unless they tell you later on, you never know whether and, if so, how, points made during an attempted overt persuasion encounter register and trigger thought, reconsideration of previously held views, etc.  Clearly there are exceptions.  

The potential the Shallal style group interracial staff discussions has that intrigues me is not the possibility of overt, publicly acknowledged, in the moment persuasion on matters of belief.  It is the experience of actually being in the presence of a diverse group civilly discussing and listening to one another on these issues. 

It's more that experience, alien to many people in segregated societies, rather than overt in the moment persuasion of one person by another on a matter of self-conscious belief, that may have beneficial effects.

Especially if it is not a one off experiences but is part of a workplace or other institutional culture, deliberately cultivated because running counter to dominant day to day realities in a multi-tribal community or society.

These are just my observations, not attributing them to Shallal or authors of works on tribalism I have mentioned.

People want to think that things have improved regarding race. This despite the GOP and the Klan working hard to roll back the clock. Blacks focus on race because it is a survival mechanism. You have to realize what is going on.

The data on the impact of racism on birth statics in the black community is chilling

NYT

https://www.nytimes.com/2018/04/11/magazine/black-mothers-babies-death-maternal-mortality.html

NPR

https://www.npr.org/2017/12/07/568948782/black-mothers-keep-dying-after-giving-birth-shalon-irvings-story-explains-why

Edit to add:

Corrected link to the WaPo article

https://www.washingtonpost.com/local/we-cant-avoid-the-subject-of-race-because-were-never-able-to-leave-it-behind/2018/05/15/9bbf6a46-5541-11e8-9c91-7dab596e8252_story.html?utm_term=.25ed848cc4ce