I have these thoughts too, but in fact that’s what the Onefinity Creator did. It allowed him create a very light Z Slider with small rails. But it’s exaggeratedly light. And first of all, the spindle mount has only one linear bearing per side and has practically no vertical height. That’s the reason why there is no big plate for spindle mounts, the leverage force would be too high.
As far as I can imagine, when building your own, keeping the mass as close to the rails (and also as close to the X Rails I would suggest) as possible would require to exactly know and measure the parts one wants to use. I find that pretty demanding, you would need to buy hardware in advance just to estimate its possible use.
Usually Z Slider kits don’t seem to worry that much on keeping a small distance between the mass and the rails:
https://www.sorotec.de/shop/DIY-Kit-Z-Axis-Alu-Line.html
https://www.youtube.com/results?search_query=Sorotec+DIY-Kit+Z-Axis
PS: I think that the considerations made in this older thread have become more up-to-date with the advent of the X-50 rails. I believe that on Onefinity CNC, the torsion of the X rail pair is also something to consider because leverage force not only puts load on Z bearings but adds on X rails too, and new X-50 rails seem quite impressing now.
What I imagined among other things is: What if there were two 80 mm mounts, one on top of the other (thereby four linear bearings instead of two for the Z slider), but the Z rails would need to be much longer in order to retain Z travel length, but for the rest, leaving the Z Slider design as it is? This way the uniquely small distance between the milling motor mass and the rails would be retained.