cardboard adolescent

Because if there's no bottom, how can any proposition be said to be "true"? It would only be in reference to some other proposition, and if there is no "bottom" proposition that is self-evident that all the other propositions can refer back to, then you essentially just have a bunch of propositions that are interconnected but none of which can be said to actually be true. Say you're building a house for instance, you have to make an assumption that there is a such thing as a wall. But this assumption is not self-evident, and if you look closer and closer you find that a wall is just a way way way generalized concept that doesn't actually have to do with the physical nature of a structure, but rather the psychological context in which certain structures present themselves, etc. So you have to make an assumption like walls exist, but if there is no bottom to our representations then any assumption you make is unprovable, and hence all the other propositions in your system which rely on this assumption are also unprovable--they are only provable in the context of your system, which makes no guarantee at corresponding with reality.