Because people in similar race or religion or area of living or language will tend to congregate in the same community
whether it is a fish market or a church mass or a wedding or any social event that that have attended.

A Dusun guy won't be bother to attend or be invited to a Iban social event.
Same goes to Chinese that wouldn't have the chance Malay Ramadan Tarawikh
Unless they are co-workers or same place of education.

A Malay will help a Malay especially when their social status is higher or sometimes lower than others.
Other Bumi ethnicities shares the same mindset of helping the less fortunate in your community.

Perhaps Chinese have different look into this but I doubt that.
I've attended around 30-40 Chinese weddings, countless CNY visits and BBQs and one of my ex is Chinese.
Perhaps this is why Chinese does not think the way Malays or others think. (I'm not saying one is right or wrong, just different way)

I cannot speak in regards of Indians because I rarely encountered them in my life to be able to make judgement

So an Iban will hire their own kind as a priority for employment. I believe Chinese too.
A Malay will hire Malay contractors for a Malay wedding because they share the same diet, religion, language
or in a shorter form, it is easier to get the job done.
A Chinese will hire their own kind because hiring a Malay would be troublesome in term of diet and custom.

Malay will have Malay patrons in their restaurant and vice versa. An Iban will buys from an Iban and such.
You get my point right?