Because it is real, and there _is_ in fact a large difference in the propensity to do this across cultures.
This post is about Switzerland, and as said by parent a lot of this is about Germans in Switzerland.
Are Germans in Switzerland more prone to hiring another German rather than a French person, or Swiss person? I'm sure that such bias exists. But that bias is nothing compared to e.g. tendency for Indians to hire other Indians. Now of course some of this can be explained by economic opportunity. The extra benefit Germans can provide to other Germans by giving them Swiss job is smaller than for Indians.
However that only explains part of it. If that was all, then Chinese people should bias much more to hiring fellow countrymen than Japanese and Korean people, while the latter two should be similar to each other. This is definitely not the case (note that we're talking about immigrants here, not 2nd+ generation).
I'm sure there's been research on this subject, and there will be some cultural trait that proxies for how much immigrants from country X bias towards hiring others from X.
I can even give you a proxy for funsies: embassies. Look at the employees at the embassy of country X in country Y. How many of them are from country X and how many are from Y? Now compare that across embassies. You'll see a lot of similarities with what I've sketched. Sometimes you'll see that both the embassy of country X in country Y, as well as that of country Y in country X (the other direction), are both primarily staffed by people from country X! In those cases it's common that country X has a much stronger bias than Y towards hiring people from their own nationality rather than based on aptitude.