Why not to add year of disbanding at band infos?

Add it to the additional notes, if you think it's necessary . I think this has been suggested before.

I think it is more important than just to add it in additional notes, if there is year of formation, why not year of disbanding?

Probably because they can reform too.

Just add it to the additional notes. Contrary to the year of formation, year of disbanding might be a lot more difficult to determine. Also, as the above poster said, they might reform.

You could just put it in notes, but why not just put like "year formed - Disbanded" and then put (ex.) 1982-2001 and if they reform, just put (ex.)2003-present