Then let me phrase it differently. With a centre-tapped layout, you only have one rectifier voltage drop, with a bridge you have two. This is obvious.
But if you look at the transformer, it's a different story.
Let's say you have an normal EI transformer.
Copper cross section of the primary is 1 cm2, secondary also 1 cm2.
This is fine for a single secondary and bridge, and the secondary is supplying current continuously.
But if you have a centre-tap transformer with two rectifiers, the secondary cross section will effectively only be 0.5 cm2, because you need space for two secondary windings, meaning thinner wire.
Only one of the two halves supplies current at any time, meaning your resistive losses will be higher. Solution is either to live with the losses (= lower secondary voltage) or increase the size of the transformer.
Especially on smaller transformers (<100 W), copper losses can be significant and higher than the extra diode drop.