It is possible that the visible effects of the curse manifest differently than the systemic effects-- as an example, consider the progression of the effects of the bite of a "Brown Recluse" spider: A spot of necrotic flesh progresses outward from the site of the bite.
This often reaches alarming proportions, and may even lead to the loss of a finger or toe, or more than one, and the progression of dying tissue may even lead to the loss of a hand or foot. [I have seen old recluse spider wounds over three inches in diameter, on the lower legs of rural residents.]
It can quickly become a hideous wound, that often resists conventional healing. Viable or healthy skin may not form over the wound, and at times, even skin-grafts will not "take" on, or over, the original wound. A physician even stated that cutting away the bite often had no effect on the necrotic ulcer, later.
Importantly, venom did not remain isolated at the place of the bite! On the contrary, a physician noted that analysis of the victim's blood showed venom proteins had quickly spread throughout the victim's body; remaining active throughout the body for a long time.
When I read of the withering-away or "mummification" of Dumbledore's hand, I had assumed the curse was proceeding in the fashion of a Brown Recluse bite; only instead of an organic protein-based toxin, it had involved a "magical toxin-like counterpart."
The withering of Dumbledore's hand had only been the visible manifestation of the "curse-toxins." Like the spider-venom counterpart, as well, a major quantity of "curse-toxins" had already proceeded up the arm, and were circulating throughout Dumbledore's body.
I had taken Snape's saying: "I have trapped the curse in one hand for the time being." had merely applied to the visible signs of the curse upon Dumbledore and his hand.
I'd thought about why amputation hadn't been mentioned, too, but since Dumbledore and Snape hadn't talked about having Dumbledore reprise the famous old Celtic Magister's role, right down to the wearing of a precious-metal hand, I'd assumed the wound would inexorably continue to "mummify," no matter how high-up (how far from the "mummy-hand") they chose to amputate.
I'd simply assumed it had been considered, and then rejected, and was not something I need to bother about. I'd been going under the assumption that, between the pair of them, Dumbledore and Snape had possessed vast magical repertoires. If they'd not followed a course of action, it was obviously not something worth my effort.
These men were the brightest and most cunning magic-wielders of their world; of ANY time. Between them, they had continually conducted Voldemort's actions, almost from the very beginning, even having discovering Voldemort's creation of an "Inadvertent Horcrux." They had done this even from beyond their graves, and their machinations had ultimately led to the destruction of Voldemort at the hands of the same "Inadvertent Horcrux."