It only works close to symptoms onset for the same reason Tamiflu only works against influenza at symptoms onset.

By the time symptoms show, for the near-totality of people, the immune system is very close to effectively neutralize the virus as it pops out of the bloated infected cells. Viral load is close to peak too. Alot of damage is already comited no matter what. Tons of infected cells will die no matter what, causing a cascade effect that will make you feel bad, maby very bad, possibly do lasting damage or kill someone vulnerable.

However, there is a still a considerable amount of damage that isn't comited yet at symptom onset, but not much a few days after symptoms started.

By that time, your immune system is not letting many new cells getting infected. The reason why the patient keeps deteriorating is from damage already comited. Cells that are already infected will die anyway, and the sludge of dead cells all over the place is causing damage itself.

The virus that is left is floating in a neutralizing soup. It's on its way out. Of course the PCR tests will detect alot of ( deactivated ) virus, but it can't do anything. A week after symptoms, it's near impossible to culture SARS-2 virus from a patient's sample, regardless of what the PCR test says or improving vs degrading health.

Studies "proving" Ivermectin or Hydroxychloroquine "don't work" are all based on witholding treatment untill the patient is deteriorating, usually over a week after symptoms onset.

Using Tamiflu like that would also show it to be "useless" against influenza.