Why can't they slowly push more dirt into the pit filling it up as they pump out and filter/treat the water?
All adding dirt would accomplish is to take up space made from pumping out water in the first place.
First and foremost because "they" cashed out all their profits over seas and bankrupted the company, neatly avoiding any of the responsibility of dealing with the repercussions of their actions. This is the most basic strategy of mining companies everywhere. They. Don't. Ever. Clean. Up. Their. Messes. If they did, most mining wouldn't be profitable. This is why people get up in arms about the DAPL pipeline and other such projects.
See also Iron Mountain Mine in CA. It's a Superfund site and it generates pH -3.6 water (which is absurd).