Here's why:

* Simplicity: A beam bridge is essentially a single, horizontal beam supported at both ends. It's a straightforward design with minimal components.

* Ancient Origins: Beam bridges have been used for millennia. Evidence suggests they were built as early as the Stone Age, using materials like logs and stones.

* Ubiquity: Even today, beam bridges are common, especially in pedestrian bridges and small spans.

Other contenders:

While the beam bridge is the most likely candidate for the simplest and oldest design, other contenders include:

* Log bridges: These are basically simple beam bridges made of logs.

* Stone slab bridges: Ancient cultures used flat stones to create bridges over small gaps.

It's important to remember that bridge design has evolved significantly over time, and many historical bridges used combinations of these basic principles.