What a good question! Why can’t I search my attachments from withing Gmail?
I think I have an answer, though people will not like it.
I once worked as a project architect for an e-Health project. This was circa 1999 and we were trying to automate exchange of data between laboratories using some kind of specialized email application. I had written a customized email client that would received the data in XML form and present it appropriately (now, you could do the same thing with XSLT in minutes).
I ended up quitting the project. One reason for it was the strong desire for users to send the medical data using Word documents. This is extremely annoying to someone who is working on data schemas to facilitate automated interchange. Basically, secretaries love Word (unstructured data entry), and they hate structured data. And they probably have good reasons too! The worst thing came down when people forking the money decided that we should really fully support Word documents. I really gave up at that point. Try as I may, I could not convince them that storing the data in Word documents was incompatible with a smart data interchange format.
So, why do I think that the GMail team finds that fully supporting attachments is not a priority?
Because they think that exchanging documents via email is inefficient and will probably go away, at least for serious tasks, in the near future. If you document has any kind of serious content, it should not be archived in your email system. Seriously. Post it on an intranet, on a Web site, using a collaborative editing tool, and so on. These solutions are not mature enough for you? It could be, but they soon will be.
Email is for 10 liners, no more. Email is not appropriate for sending large documents, publishing your essays, and so on. The only reason I use emails for other purposes is because there is no other way, but there soon will be.