I mean, he did. Such poison damage is a bit similar to bad chemical or fire damage, except it often affects deeper tissue stronger and can "hide" behind a relatively healthy surface skin.

The damaged deep tissue has died and turned to slush from the poison, which the body needs to dispose of and then rebuild from scratch. However, with large damage in the deep tissue, which is where skin repair should originate from, this is often impossible to achieve perfectly. So you often get enclosed pockets of "weird/wrong" tissue that has no proper drainage to the outside or inside to the lymphatic system. So occasionally, bacteria or viruses crawl in and fester. The bad circulation due to the previous damage means the immune system has a hard time fighting it, which usually leads to it cordoning off the area, forming a so called abscess. If this abscess can then be properly drained to the surface and sterilized(!), this can keep it calm for very long times and is comparably easy to manage and monitor.

And there is no good alternative. You can try and remove all the damaged tissue, which has the unpleasant side effect of having the surgeons carve a huge hole in you. Which the body again will have to try and repair, including massive scarification, possible los if function is nerves and/or muscle tissue is lost and a very high risk of formation of more abscess-prone internal scar tissue. If there's no way to deal with the occasional infections in a hygienic manner, there's a high risk of abscesses draining internally sooner or later. This almost always leads to intense sepsis, which is very often lethal after mere days.

Which is why the usual alternative to large-scale deep tissue damage is called amputation, even today and with all the crazy medical advances we have.