Here's why:

* Concentration Gradient: This refers to the difference in the concentration of a substance between two areas. Molecules naturally move from areas of high concentration to areas of low concentration, attempting to reach equilibrium.

* Passive Transport: Many molecules enter cells through passive transport, which doesn't require energy. Examples include:

* Diffusion: Movement of molecules down the concentration gradient.

* Osmosis: Movement of water across a selectively permeable membrane from an area of high water concentration to an area of low water concentration.

Other factors that can influence movement:

* Membrane Permeability: The cell membrane's structure and the properties of the molecules involved can influence how easily they cross.

* Temperature: Higher temperatures increase the rate of molecular movement.

* Pressure: Pressure gradients can drive movement, particularly in the case of fluids.

* Active Transport: Some molecules require energy (from ATP) to move against their concentration gradient. This is called active transport, and it requires the involvement of specialized proteins.

In summary: While other factors play a role, the concentration gradient is the primary driver of the net movement of molecules into cells, especially in the case of passive transport.