Because Space dont have form, what we can understand. It is so huge, and this huge is in something. We can only say, that Space is place which in is material moveings-process, and these processed makes everything what we can sense. We cannot sense non-material things. And because we cannot sense it, because our senses are material thing, we cannot say, what kind is it.
Science can only be basic at material space, which dont have form, size, born-time.
We humans are same. Believe or not. We can only speak and do that what we can see. When i die, i see, what happens. Before that, we have lot of continues, endless work to to.