Because that “multiple” doesn’t refer to multiple hardening (σκλήρωσῐς/sklḗrōsis) events, but rather to hardening as something uncountable happening in multiple spots.
It’s roughly “multiple hardening”, note how the lack of a plural doesn’t feel off in this one either.
For reference, in languages showing adjective-noun agreement, the adjective gets the singular (e.g. Portuguese “esclerose múltipla” - the plural would be “escleroses múltiplas”).
Because that “multiple” doesn’t refer to multiple hardening (σκλήρωσῐς/sklḗrōsis) events, but rather to hardening as something uncountable happening in multiple spots.
It’s roughly “multiple hardening”, note how the lack of a plural doesn’t feel off in this one either.
For reference, in languages showing adjective-noun agreement, the adjective gets the singular (e.g. Portuguese “esclerose múltipla” - the plural would be “escleroses múltiplas”).