Why don't we realise space instead of Nibbana?

66. Modes of Production “What is it that is kamma-born, what cause-born, and what season-born? What is it that is none of these?” “All beings, O king are kamma-born. Fire, and all things growing out of seeds are cause-born. The earth, water and wind are season-born. Space and nibbàna exist independently of kamma, cause and season. Of nibbàna, O king, it cannot be said that it is perceptible by the five senses, but it is perceptible by the mind. The disciple whose mind is pure, and free from obstructions, can perceive nibbàna. http://www.buddhanet.net/pdf_file/milinda.pdf