What's so bad about building XML with string concatenation?

Question

In the thread What’s your favorite “programmer ignorance” pet peeve?, the following answer appears, with a large amount of upvotes:

Programmers who build XML u

Answer 1

I think that the problem is that you aren't watching the xml file as a logical data storage thing, but as a simple textfile where you write strings.

It's obvious that those libraries do string manipulation for you, but reading/writing xml should be something similar to saving datas into a database or something logically similar