Why is String.Concat not optimized to StringBuilder.Append?

Question

I found concatenations of constant string expressions are optimized by the compiler into one string.

Now with string concatenation of strings only known at run-time,

Answer 1

I believe it would be a little too complex for the compiler writers. And when you are referencing the intermediate strings inside the loops besides the concatenation (for example passing them to some other methods or so), this optimization would not be possible.