Because many women simply aren't aware they can have a homebirth, or think they need permission to have one.
Many women equate homebirth with freebirth and don't realise you get midwife support or can transfer to hospital if need be. Many don't realise you can have gas and air and assume you don't get any pain relief.
Many people don't understand that homebirths are generally as safe as a hospital birth for most babies of low-risk mothers especially for subsequent banies or that mothers generally fare better at home than in hospital wrt intervention.
Because we've been conditioned to think that pregnancy is a medical condition and not a natural state of being, and therefore a hospital is the only safe and sensible place for all women to give birth regardless of their circumstances.