为何省略了定语从句中的who are

The procedure is so  commonplace that 1.5%of the babies born in Western Europe,North America andAustralia owe their existence to the technology .

这个句子中babies 后面跟的who are born为何省略了?

The procedure is so  commonplace  that 1.5%of the babies born in Western Europe, North America and Australia owe their existence to the  technology.

黑体部分为形容词短语，它本身就能作后置定语，这是非常正常的表达。为什么要用复杂的定语从句呢？当然从理解角度，它可以扩展成一个定语从句。即使是定语从句省略成分，省略的是 who are，不是who are born。