Well, the Catechism doesn’t quite say what you claim. Divorce is a disorder, it goes against the natural law, but:
The full rich text is very good and covers both where it is sinful…and where it is tolerated.
On the one hand:
Divorce is a grave offense against the natural law = sinful…(noting that it is indeed an offense against the natural law even turning up the heat…for those without the benefit of Divine Revelation can discover that it is immoral via reason etc)
2385 Divorce is immoral = sinful (the very meaning of immoral)
The text you quote is referring to when it is can be done …and note it says:
If civil divorce remains the only possible way of ensuring certain legal rights, the care of the children, or the protection of inheritance, it can be tolerated and does not constitute a moral offense
Thus then…civil divorce does not constitute a moral offense…whereas otherwise it can be a “moral offense”…
I of course understand that many have had to get civil divorces in ways that were not sinful (I refer to the act of getting a civil divorce)…as described above.
I understand where some can be as aware of the reality of divorce…for their own civil divorces fit under what is described above in the Catechism…or they have friends or family in such situations (fitting the above quote).
(and of course I pointed out it has been found by the Church that they were not married in the first place…then they were never married to begin with before God.)
And that being their personal experience with the idea of civil divorce …where it was “tolerated” as the only possible way of XYZ (see CCC quote above etc) in their married but separated state…they may not realize when it is sinful…