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

Probably because it's complicated to match such a pattern in the code, and in case the compiler can't do the match for some reason, the performance of the code is suddenly terrible. Optimising code like that would encourage writing code like that, which would even further increase the negative impact in the cases where the compiler can no longer do the optimisation.

For concatenating a known set of strings, StringBuilder is not faster than String.Concat.