Because that this has to be supported for everybody, and the main site can't handle it (as was discussed), and no mirror volunteered to take the search load for the whole php.net, so for the meanwhile google is used until a better search than htdig will be implemented... At any case they're not returning to htdig AFAIK, so, I think it should be removed... and even if they do, remove it until then...
I was thinking about exactly the same thing. I have removed the search information. In the future, either a livedocs or an mnogosearch based search will be available on the mirror sites (or a combined one).
Goba