1)Multicellular organisms aren't random collection of cells.They have cells,which are specialised in tissues and tissues are collectively placed as organs.Simple cell by cell division is not possible in these organisms because of the specialisation of the cells.

2)For producing individuals from regeneration, the organism has to be cut up or broken up.And it can't depend on being cut up and reproduce more of its kind.Also complex organisms have specialised cells for reproduction process and hence there is no need of regeneration. Those multicellular organisms are not simply collection of cells,They are arranged complexly.

Hope this may help you.