Because if we have multiple perspectives open to us, we can choose which one to adopt (or which bits of each, of course).
Analogy: why should there be more than one clothes dye? The State could just decide a colour, say navy blue, and require that all clothes were navy blue. Nobody needs more than one colour, after all. Take red dye: why is it more valuable, just because it's different? It's not! What's valuable is when red dye becomes available, because then we can choose what colour clothes to wear.
Even if the State could provide the perfect life for everybody, in the sense that all the 'correct' choices for that individual have been made... it is still better to let the individual take the risk of making a mistake.
However, whether this applies to language death is still debateable. For a language to be valuable in this account it would have to a) contain a worldview that is notably distinct from its alternatives, and b) have that worldview be an accessible choice for a significant number of people.
The latter is the problem with languages that become too tied to the material culture of the speakers: accepting the language becomes accepting the material culture, and when that material culture is so... defeated by other cultures, yes it's still valuable to have the option of living in the neolithic open, but it's more valuable to concentrate on possibilities that some people might actually accept.
The former is an assumption that people make, but I don't see why it's true necessarily. Many languages in close proximity to each other are pretty similar, and I don't see how they encode significantly different worldviews from one another. They do, of course, encode social differences (ie group membership), but those will become irrelevant as the groups change anyway. So it may be that speakers of endangered, similar language (not necessarily 'genetically related' ones) would be 'better off', morally speaking, if they consolidated their score of non-viable very-slightly-different perspectives into a single language with the critical mass to actually be sustainable.
But the river tripped on her by and by, lapping
as though her heart was brook: Why, why, why! Weh, O weh
I'se so silly to be flowing but I no canna stay!