We don't but some people do. Or think they do, but that's the same. It's not like I'm losing something because I can always use the simpler filesystems myself. I went with ReiserFS for over a decade: never really had to think about my file systems which is what I want, after all. Once you find a good file system, you stop thinking about file systems.
For me, this means two things.
I want my file system to be safe and transactional: either my change gets in or it doesn't, but I don't want to find my file system in some degraded in-between state. Ever. I'm willing to pay for that with cpu time or i/o speed, that's like the first 90% of my requirements.
The second criteria is that it's generally lean and doesn't do anything stupid algorithmically. It should support big files, provide relatively fast directory lookups (so that 'find' will run fast), and have some decent way of packing files onto the disk that doesn't defragment the allocations too badly, and ideally do some book-keeping during idle i/o so that I don't really have to run a defragmenter, ever. But these are kind of secondary requirements that aren't worth anything unless the file system keeps my files uncorrupted and accessible first and foremost.