I should probably start by saying that I had an immediate need to flatten a 33" x 10' slab of claro walnut. Other than that there were several reasons I built what I did and the way I did:
First off I have a very small shop and have no outdoor paved area to work on. My shop space is so small that there isn't room to set up for a 33" wide 10' long slab, other than in the same space as my table saw and extend from there into the open area of my shop. The saw wasn't wide enough to use as a surface to support a couple of 2x4's, nor was it long enough. That lead me to the idea of making something that could be repeatedly put together and taken apart for storage.
I suppose I could have made what I did out of wood, but over time it would likely warp and move a bit. I've never had any 2x4's, even kiln dried ones, stored for several years and have them stay straight and true. That led me to think about using 2 x 6 metal studs, but as I thought about the other details I eventually settled on square and rectangular tubing. I have nearly 150 slabs, almost 90 of which are walnut and similar in size to the test slab I flattened. There is no way a wood jig would hold up the the wear and tear of flattening even half that many slabs without being rebuilt.
Perhaps the best reason for making it out of metal is because I can. I have a TIG welder and a metal cold cut saw so in some ways it is just as fast for me to build in metal. The time consuming part of my build was the need for it to be bolt together and I could have simplified that part but I did it the way I did so I could easily assemble it by myself and easily change the length of the rails as well as substitute different height rails for thicker slabs.
Before starting my build I think I looked at every Youtube video and every photo and video on Instagram of any kind of slab flattening set up. One thing I noticed was that a lot of guys that do this for a living or at least as a money making gig have some sort of wood /metal mixture to their builds or all metal. That told me that others had also done a cost benefit analysis and come the the same conclusion as me. My goal was to make something that was simple to use as well as sturdy and maintenance free. In some ways my Granberg mill inspired at least the spirit of my build. It is amazingly simple, reasonable sturdy and maintenance free. And just like with your question about why not use wood, there are ways of building a milling jig out of wood blocks and all thread, and I have no doubt they work and meet the needs of those that use them, just as I'm sure your wood sled and 2x4 flattening jig meets your needs.
