Cuz there isn't much of it and it's incredibly cheaper and easier and safer to just bury it in the ground on site.

Source.

" Little waste is generated. Nuclear fuel is very energy dense, so very little of it is required to produce immense amounts of electricity – especially when compared to other energy sources. As a result, a correspondingly small amount of waste is produced. On average, the waste from a reactor supplying a person’s electricity needs for a year would be about the size of a brick. Only 5 grams of this is high-level waste – about the same weight as a sheet of paper. "