It is how it is (of necessity) for various reasons.

Each archive is a separate database in its own right, with its own index.
Firstly there is a max number of terms in an index of 1,000,000. Once an archive gets to 150k posts it:
1. Won't index everything properly
2. Will take so long to search that smaller devices will time out, and the server load will increase to the point the BB as a whole will slow down
That limit used to be much lower, hence Greg had to break down the early archives into very small chunks.
While some of those could in theory be combined into large ones now, the physical addresses would change and all historic search engine references would be lost.

When you do a google search and find a link that sends you to the archive index page, that should be either:
1. A post that has only recently moved from the live board to the archives (this happens automatically every morning), or
2. A very old post that was originally indexed when it was under Greg's old file structure and google never considered it important enough to re-index

For the vast majority it should work properly though......
Google this for example - "david spragg chipboard hard rock guitar" - when I do so I correctly get posts I made back in 2004 and they load properly.

The searching across multiple forums was an option in the software that we tested a few years ago but it presents the same server load issues as above, and would need another instance of the forum running in order to display results from multiple databases. We really had to let this go as a trade off against being able to retain all uploaded images indefinitely. In Greg's day 14 days was the max for all images.