Having a limit helps save memory/IO/CPU cycles, reduces the strain of having to render them all in a browser, & against a server it also reduces bandwidth. But of course there could be situations where you might not care or 100 doesn't begin to affect performance - thus there is an zimbraPrefMailItemsPerPage attribute which can actually go higher (though it doesn't add it to the user selectable drop down - it just applies it). It's a 'user-beware' approach to ignoring performance norms, increase it to your own liking; if we didn't fix it to a max of 100/drop-down in the end-user UI it'd be prone to hi-values and forgetting that's the cause of slowness. And that only makes a source of negative feelings towards a product when it becomes slow to navigate. Perhaps at some point we'll mimic some other clients and give a false bottom (then an option to click and add more to the view as you scroll down) - it's safe to say that we're always looking to better it.
You can vote for
Bug 7411 - allow display of greater than 100 (or arbitrary number) items per page (title, description & component conflict - basically wherever we can increase it we will - that bug kinda teeters between user web-client and admin console web-client). Though most just really want something like
Bug 4175 - Permit the selection of all messages in a folder so you can take action on lots of mail items at one time, but implemented as a 'select all results'.
Zimbra's indexing is very powerful:
Search Tips - Zimbra :: Wiki (examples of the queries possible)
Many find it handy to set preferences > general > show advanced language in the search bar.
I know you're saying 'tedious to search through emails especially if you don't know who it was from or what it was about' but you must have some idea on what your hunting for - most don't sit back and admire 3k emails at random. You could open a RFE for something like a
mail-trends or
Xobni in the web-client, we did did have a start of a
Social Zimlet that's not being pursued at the moment because of license restrictions, to show traffic & connections across a search (plus is:anywhere & a big enough max results to cover across all your mail would be a little much for it).