It's because the first vowel in 'about' is an unstressed schwa. It sounds unnatural in English to stress or lengthen a schwa.
(The only exception I can think of is 'evennn'. e.g. Bob and Sue are at a dinner party with friends. Bob tells everyone that he likes all vegetables. Sue knows he doesn't like broccoli, so she nudges Bob and says "evennn...?". This makes sense if 'even' in fact contains a syllabic nasal consonant rather than schwa: /'ivn=/.)