Probably for the same reason that he spends money on expensive cars and big houses - because he can.
Just a bit of additional information. Very few people receive custodial sentences for a first offence of driving whilst disqualified. There has to be considerable aggravation (such as, for example, being caught driving away from court following the disqualification, or stealing a vehicle, being involved in a police chase, causing a bad accident, repeatedly offending, that sort of thing) before custody is considered.
It is completely wrong in my view. Driving whilst disqualified is a deliberate act which the driver has chosen to undertake contrary to a court order. It is not something you can do "recklessly" or by accident. But as I said earlier sentencing guidelines have gradually suppressed sentences available to courts for many offences in recent years.