Shields Shall not be Splintered

Because they're not made of wood. Such arms are made of leather.   After having been boiled and macerated, or rendered perfectly soft, is moulded, stamed or otherwise worked into a tough, rigid form. While hardening of its own accord, leather becomes like wood or horn when backed with varnish or shellac.

Unlike wood, which can splinter or crack upon impact, armour made of boiled leather offers flexibility and resistance to damage, absorbing and distributing the force of a blow more effectively. This makes leather shields less prone to breakage and provides more reliable protection over time. Moreover, cuir bouilli shields weigh less and are easier to wield and maneuver in combat, permitting warriors to react quickly to incoming attacks. As such, only those cultures unfamiliar with this technique of transforming leather would use wood as an alternative.

Felt it needed saying.