Why optimus prime changed semi truck into old truck in transformers 4?

In "Transformers: Age of Extinction," Optimus Prime transforms into an older model semi-truck to blend in with the changing world and avoid drawing attention to himself and the remaining Autobots. The shift symbolizes his adaptation to a new era, reflecting the decline of the Transformers' presence on Earth after the battle in Chicago. The choice of an older truck also represents a connection to the past and the resilience of the Autobots despite their struggles.