In Subspace protocol, due to use of Proof of Archival Storage consensus, security of the protocol is directly proportional to space pledged. As more space is pledged to the network, it becomes harder and harder to mount 51% attack because you’ll need to have a lot of plotted space to do so and can’t build longer chain quickly with little space because of Proof of Time protocol that runs in parallel.
So for security reasons we would like space pledged to grow as fast as possible to as high level as possible, this makes “average” space pledged of the protocol at any point in time higher.
As for the target of 8PiB it was based on something that is attainable relatively quickly, but not instantly to give smaller farmers time to plot, etc. Yet everyone is interested to plot and start farming as soon as possible because this will both unlock rewards sooner (currently weekly allocation for farmers is paused until we reach that target in space pledged) and they will be the first ones to get rewards enjoying smaller competition (see Gemini3g space pledged dynamic - #9 by dariolina for space pledged dynamics over time and how high the peak was on 3g, we expect it to be even higher on 3h).
I hope this makes sense.