請問這句 Most universities across the world now have a good proportion of students enrolled in their schools 為什麼不是enrolling in their programs?

enroll是「作格動詞」(ergative verb）。所以即使在做「不及物動詞」用時，在邏輯上，也是「被動的」意思。
An ergative verb is a verb that can be both transitive and intransitive, where the subject of the intransitive verb is the same as the object of the transitive verb. For example, 'open' is an ergative verb because you can say 'The door opened' or 'She opened the door.'
see p.519 Collins Cobuild English Dictionary for Advanced Learners
If you "enroll" or "are enrolled" at an institution or on a course, you officially join it and pay a fee for it.
see p.511 Collins Cobuild English Dictionary for Advanced Learners

Is it too late to enroll at the college?（不及物，無受詞）
I enrolled for/in/on the modern art courses.（不及物，加上介系詞，才能接受詞）
He is enrolled as a part-time student.（及物，被動態；主詞he是動詞在邏輯上的受詞）
They want to enroll their children in their local school.（及物，有受詞children）
see p.406 Cambridge Advanced Learner's Dictionary (2003)

一般動詞的「過去分詞」當「修飾語」或「補語」時，屬及物動詞的過去分詞除了「被動」外，還有「已完成」的意思，如果是不及物動詞，則只有「已完成」的意思。

Most universities across the world now have a good proportion of students "enrolled" in their schools.
這句裡的「受詞補語」用「過去分詞」enrolled比較合適，因為註冊入學是「已完成」的狀況，較能搭配主要動詞have的邏輯（表示現在已經有了這個結果）。此外，作格動詞的enroll不需要考慮它的分詞是主動或被動的關係。
述部＝動詞have +受詞a good proportion of students＋受詞補語enrolled in their schools
這句的主要動詞have，不屬於「使役動詞」（causative verb）。

My friend and I had our horses "stolen" from our barn.這也是一個過去分詞當受詞補語的例子。
這裡的had不是「使役動詞」，因為受詞補語的該行動非主詞的意志。

I have my laptop "repaired."這裡的have才是「使役動詞」的用法。受詞補語的行動為主詞的意志。

一般規則，分詞作形容詞用的時候，主動與被動決定是現在分詞還是過去分詞。在英語世界裡，入學是被動的。一個學生是被接受入學，或是被接受註冊。所以是admitted to xxx University; accepted into, enrolled in their school. 美式思維是：學校選學生，而不是學生選學校。這點和中國式的想法正好相反。

My explanations: "have......enrolled" that is the verbal phrase with present perfect tense to represent already completely put on a list or register; While "Schools" that are plural proper nouns represent department or division of most universities for study of several particular subjects. Yip