It's a matter of pride, supported by tradition. It's often bandied about (semi-seriously) in Australia that the second-most influential person in the country - aside from the PM - is the Test captain. It's entirely unthinkable that someone would turn down the Test captaincy like Mark Butcher did if it was offered to them. The formula for consistent captaincy policy started with Border, after it became painfully obvious that the Hughes/Yallop selection cluster**** had gimped the team beyond repair. If one's captaincy was predicated on results, Border would have been stripped of it well before he turned the side around. After sticking with Border yielded serious results, it was agreed that the captain's authority - bolstered by length of stay - was vital to a winning side. It's why they've never dropped a captain on results since then and even refused to drop them on form. It's not a flawless policy, but it correlates very tightly with Australia's winning period.
A Test captaincy nod in Australia is a permanent invitation to lead and the idea of another captain existing in the same team (and dressing room, and press conferences) as Ponting would cause a natural division of loyalty as to who the leader - as opposed to the titular captain - of the side was. That would be nothing but destructive considering how long Ponting's been leading the side and nurturing most of its personalities - who would probably flock to him if there was a split in the team. With the weight attached to Ponting's legacy, there's every chance of a Pakistan-like split should things go **** up.
In any case, Ponting, for all his tactical incompetence, is an exemplary leader of men and unusually productive with the captain's knock. Captaincy isn't completely about setting fields; even Brearley attested to that in The Art of Captaincy. It's probably the easiest failing to delegate, as well - Warnie set an awful lot of Ponting's fields before he retired, and I'd hazard to guess that it's not just Ponting within the team who's out of ideas now re: field placements.