وَلَمَا كَانَتِ الِاسْتِعَارَةُ أَبْلَغَ مِنَ الْحَقِيقَةِ
Otherwise, how would metaphor become more emphatic than a plain literal expression?
“Zayd is brave like a lion” keeps the two subjects distinct. “Zayd is a lion” can make a stronger rhetorical claim of identity. On the intellectual account, the degree of hyperbole created by that claim is a major source of metaphor's effectiveness.
This is not a rule that every metaphor is always superior to every simile; fitness to context still matters. The technical issue is narrower: mere replacement of one name with another does not fully explain the intensification produced by a well-formed metaphorical assertion.