There are many reasons.
The first, clearest one, is that 32bpp is simply:
- a lot of work
- requires knowledge/willpower to learn using 3D software (unless you want to hand draw the 32bpp which might actually be a great idea due to the following points)
- constantly fighting technical issues which were not nearly as problematic with drawn graphics
* vehicles have different lengths on diagonals - you want to make one 3D model, not two ... with drawing you draw 4-8 views regardless so it is not really an issue there
* slopes are actually one huge hack - making tunnels, bridges and other slope related things is just wtf because the actual 3D height is different for those things
* everything must be with hard edges (no antialiasing) - this alone might seem like a tiny thing, but it means you HAVE to render everything a bit bigger, and then crop it in postproduction programs like After Effects for me. This just adds a ton to ALL of the problems and adds a lot of work. Things like railtypes are just "more proper" to do this way, but ground tiles do seriously BAD things if they have semi-transparent edges. Fun fact with ground tiles: Nobody has yet made a proper x4 or x2 zoom tile shape mask. (the exact pixel shape how should each sloped tile look like so they would all fit together properly)
* company colours do not support anti-aliasing - this just demolishes any nice rendered graphics unless you use the company colour mask on the whole sprite
* the resulting graphics actually look visually WORSE than classic PROFESSIONAL 8bpp graphics from 1994-1998 - and NewGRFs made in 8bpp in that fashion to fit the old graphics. Not a single 32bpp project looks good in x1 and hardly even x2 zoom at the moment. Also, zbase has set a terrible example of how not to do graphics and many authors are/were afraid to even try 32bpp.
* once you start doing 32bpp with extra zoom, the NewGRFs quickly have hundreds of megabytes so people often do not even want to download them, for example putting 300 MB RAWR on a server just means almost nobody will want to connect.
* you also need to have 8bpp sprites and converting them from 32bpp is super clunky. Unless you just want to say "f*** you" to anybody with 8bpp blitter like Pikka does with Pineapple (transparent 8bpp sprites) without even trying to make 8bpp there. (which again, is extra work) ... and actually, I think if someone finds a good way to make 8bpp from 32bpp images, it could be a really interesting project because the TTD colour scheme would remain, just with more pixels. Like the polish set shows, it fits and looks nice at the same time.
Why do we not assemble and make one nice project? Well first off there is not really anybody to assemble with, second each of us spends free time on making our own dream, and the dream apparently was not collective work for any of us. We would also have to agree on a GPL licence probably - and that means providing full source of all 3D things and I personally do not even want to hear about that idea. From huge file sizes, through each person having differently messy 3D projects, to the fact that a 3D file is a lot more reusable for your own use/or even selling 3D files, than sprite pictures. What you suggest would be very nice and obviously the most productive way, but I personally have no interest in doing that.
I would not hesitate saying that gold rush is a cute mockup but believing/relying on it actually becoming reality and released is very naive to say the least. At the same time I can say that RAWR is pretty dead - lot of work, not great result - , I have started BRIX instead which has a great visual concept. In the last couple of weeks I actually did decide I will give 32bpp/EZ graphics for OpenTTD one more try, and I am working on a completely new train set. After that is done, I will probably get RAWR to version 0.0.2 (I actually had it almost done before I stopped working on it earlier this year - 0.0.2 adds bridges and removes a s*** of bugs. Also a lot of improvements to the terrain were made), and getting BRIX done would be a dream come true. Again, all it takes is insane amount of time and effort, but what I get from it is knowledge, which makes it worth it.
now just to find the time!
Edit: the chinese stuff Leou makes is nice.