I must say I have found Google very good whenever I use it to search for an error message - though most of my searches are not for Arduino problems.
I have written a diary program for myself (on my PC) that has a simple full-text search facility (an add-on for SQLite) and it does work very well with almost zero programming effort on my part. But my diary entries are very terse and I generally know that the word I am searching for is actually in an entry. I don't think I have ever used it to search for a phrase.
I have no idea what the overhead might be to maintain a searchable list of words in a database as large the Forum's.
Google's great strength is that it gets you useful information - usually on the first page of results. Having a Forum search facility that can produce a list of 500 Threads with "MAX6675.h" without any order of priority is not necessarily much value.
I don't think "WProgram.h" is a good example because it has a very specific solution and it really does not matter what Thread is identified. When I typed that into my DuckDuckGo search bar the first result gave the solution.
...R