Why use plywood to hold the bearings for the airbrakes on a fibreglass glider?
This use of plywood is not ideal, but it works. The wood soaks up the epoxy and becomes an integral part of the fiberglass construction, and is easy to mold, so laying up the same geometry from glass fibers alone would be much more laborious.

Also, plywood is reasonably isotropic, at least in the two directions of the plywood layers. If you want to achieve similar isotropy with glass fibers, you need to interweave them - just wrapping one roving around the bearing will not do.

You could use other materials - aluminum would do, but doesn't bond as well to the epoxy resin. That Schleicher uses plywood has historic reasons. They used to build gliders with wood, some had steel tube fuselages, but wood was their primary construction material. When they started to build glass fiber gliders (the first was the ASW-12, which was a commercial version of the D-36, on which Gerhard Waibel cut his teeth as a glider designer and won the German gliding championship in 1964) they used what they knew best. And it worked.