Besides those reasons already mentioned, there are a few more hurdles with offering high grade "drop in " replacement wood grips for Beckers.
The first is COST. Besides covering CNC machine time, the cost to "stock" a Becker in truly high grade wood is expensive. Why? well, because most of the knife scale providers don't offer them in sizes large enough to make the huge Becker handles. The real problem is the width. Most knife scales or Bowie blocks are only 1.5" wide. Beckers need at least 1.8" to have enough wood to make a factory replacement set. Raw unmachined high grade , stabilized slabs sets in such a large sizes would run $30 wholesale before any machine work and finishing was done.. Add machine time, finishing. the cost of rejected peices (guaranteed with wood) and reasonable profit and you are looking at $75-$100 for such a set of scales depending on the wood species..
The second issue is the durability of bolt on solid wood handles over time.. Any kind of bolt on knife slab can work loose over time. Combine that with the vibration and shock by the hard use many of the Beckers endure and that is prescription for wood breakage. To me, installing a set of high grade wood handles on a Becker is a lot like installing a stock on a hard kicking rifle. It has to be installed perfectly and stay that way for the wood to hold up under the shock it will be subjected to .
My recipe for a wood handled Becker is a relatively flaw free, high quality set of stabilized slabs that are permanently installed with Accraglass and 1/4 shouldered bolts (Corby, Acorn ,Loveless etc) . Basically, that is the same way any high grade custom knife would be handled in wood . Beckers should be no different, IMHO..
And really, you look at pretty much any other brand of knife that uses bolt on slabs- Esee, Rat, Ranger, etc etc, there are no wood grip options for them ,either.