Basically because there no limits to what you can do with Ethereum
you can write a contract that leeches the resources of the network and the only way to solve this would be to limit what people can build on ethereum
( meaning its no longer able to support every program imaginable) or one could create a central entity that can remove parasitic contracts but that just ruins the whole decentral concept
This means in short that the basis for Ethereum
is flawed in a fundamentally insolvable way.