Why does espresso require 9 bars of pressure?

9 bars became the standard through decades of Italian experimentation — it's the pressure sweet spot that emulsifies coffee oils into crema, extracts soluble compounds quickly (within 30 seconds), and produces the thick, syrupy texture that distinguishes espresso. Lower pressures under-extract and produce thin, sour shots. Higher pressures can over-extract and produce bitter shots. Most quality machines regulate to exactly 9 bar (some specialty machines allow pressure profiling).