Because that is a big ‘if’! ‘If only, we can dispose off the mind, ..’, then there is the advaita-siddhi. The difficulty is exactly there – to dispose off the mind.
When our shirt loosely fits us we can take it off easily. But if the shirt is tight, the taking off might have to be made with some effort. And when we are required to take off our very outer skin, imagine how difficult it could be.
Just as the skin is sticking to our body, our mind is sticking to us, but in deeper proximity! A dirty stinking sticky cloth becomes pure when the dirt, stink and stickiness are off the cloth. It is not necessary to look for another cloth.
The same cloth, when the dirt, etc. are off, becomes the pure cloth. So also for our JIva we don’t have to look for a new entity called Brahman; if we can remove the present dirt and stink of the mind, that should be enough.
The same person will emerge as the pure Brahman. But that is exactly the formidable task – to remove the dirt and stink that is so deeply adhering to mind! Mind refuses to be disposed off.