To see why, imagine what it would have meant c. 1900. The university existed, it had a lot of students and faculty. None of them had automobiles. Many, presumably, had horses. Sustainability would have included assuring a sufficient supply of pasture land for all those horses into the indefinite future. It might have included assuring a sufficient supply of firewood. It would, in other words, have meant making preparations for a future that was not going to happen.
The same is true today. Making sure we can continue our present activities into the indefinite future makes sense only if we believe that we will be doing those things into the indefinite future. Judged by what we have seen in the past and can guess about the future, that is very unlikely. We do not know what the world of forty or fifty years hence will be like, but it will not be the same as the present world, hence it is very unlikely that we will be doing the same things in the same way and requiring the same resources to do them with.
The issue was recently brought to my attention when a colleague at a faculty meeting gave a glowing description of all the good things that were being done or planned in support of sustainability, up to and including a future teach in. I asked him one question—whether any part of the plans included presentations of arguments against sustainability. His answer was that any arguments against sustainability would be presented by speakers who were in favor of it.
That is not how universities are supposed to function.