Why new ships need sailors before commissioning

Why do new warships put pressure on sailor numbers before they enter service?

Sailors are assigned during construction to test equipment and help resolve operating problems years before commissioning.

HMS Queen Elizabeth was ordered in 2008, laid down in 2009, launched in 2014, and commissioned in 2017. A nine-person advance crew was already assigned before the senior officer who served from 2012 to 2016 arrived. By his departure, the assigned crew had grown to 580 personnel. Those sailors were testing systems and discussing how the ship would operate while construction continued. Their involvement explains why a new ship draws on personnel before entering the operational fleet.