Why don't we realise space instead of Nibbana? 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
What do you mean by "realize"? See the definition of Ākāsa here, which begins,


  'space', is, according to Com., of two kinds: 
  
  
  limited space (paricchinnākāsa or paricchedākāsa), 
  endless space (anantākāsa), i.e. cosmic space.
  
  
  1) Limited space, under the name of ākāsa-dhātu (space element), belongs to derived corporeality (s. khandha, Summary I; Dhs 638) and to a six fold classification of elements (s. dhātu; M.112, M.115, M.140). It is also an object of kasina meditation. It is defined as follows: "The space element has the characteristic of delimiting matter. Its function is to indicate the boundaries of matter. It is manifested as the confines of matter; or its manifestation consists in being untouched (by the 4 great elements), and in holes and apertures. Its proximate cause is the matter delimited. It is on account of the space element that one can say of material things delimited that 'this is above. below, around that' " (Vis.M. XIV.63).
  
  2) Endless space is called in Atthasālini ajatākāsa, 'unentangled', i.e. unobstructed or empty space. It is the object of the first immaterial absorption (s. jhāna), the sphere of boundless space (ākāsānañcāyatana). According to Abhidhamma philosophy, endless space has no objective reality (being purely conceptual), which is indicated by the fact that it is not included in the triad of the wholesome (kusalatika), which comprises the entire reality. Later Buddhist schools have regarded it as one of several unconditioned or uncreated states (asankhata dharma) - a view that is rejected in Kath. (s. Guide. p. 70). Theravāda Buddhism recognizes only Nibbāna as an unconditioned element (asankhata-dhātu: s. Dhs. 1084).


So maybe people do perceive space: both limited space (in ordinary perception) and unlimited space (as an object of meditation).