Why do animal cells lack a contractile vacuole?
Lodish et al. (2000) explain that animal cells rely on the Na+,K+-ATPase to pump Na+ out of the cell. Exporting electrolytes decreases the hypertonicity of the cell, thereby preventing too much water from entering. 

Plant, algal, fungal, and bacterial cells are surrounded by a rigid cell wall. Any osmotic influx of water in a hypotonic solution leads to an increase in intracellular pressure, but not in cell volume. hence, the cell wall effectively prevents the cells from lysis.

Protozoans do not have a rigid cell wall and these organisms sometimes contain a contractile vacuole that permits them to avoid osmotic lysis. A contractile vacuole takes up water from the cytosol and periodically discharges its contents through fusion with the plasma membrane. Thus, even though water continuously enters the protozoan cell by osmotic flow, the contractile vacuole prevents too much water from accumulating in the cell and swelling it to the bursting point.

Reference
- Lodish et al. Molecular Cell Biology. 4th ed. New York: W. H. Freeman (2000)