why do meristematic tissues have dense cytplasm and no vacuoles?

Meristematic cells have a dense cytoplasm because they are actively dividing cells and have a high rate of respiration. They have more number of organelles like mitochondria. These cells are very active and rapidly divide to form new cells, so the cells are constantly producing a lot of bio-molecules, making the cytoplasm very dense.

To make the cell division easy the cells of meristematic tissues are small, very thin walled, and without any vacuoles. Vacuoles basically serve the purpose of storing of nutrients and excess salts etc. Meristematic cells do not need to store these nutrients.This is the reason why in meristematic cells vacuoles are non existent.