Why is all construction equipment yellow?

It is not, but in a sense a recommended safety standard. Aircraft used for training were done up in Yellow and Black ( Like Piper Cubs) for safety- visibility reasons. as for Cranes, they are not all Yellow. Different makes have different ( colors of Arm) so to speak. Manitowoc- Red, Allis-Chalmers (* rare) - Persian Orange, and so on. Some construction companies have their own livery styles as do railroads and trucking companies- so the paint schemes may vary. there is no legal requirement they all be Yellow- but it tends towards safety- heads-Up!