It be worth looking up the I-V curves of solar modules on a datasheet - a key factor is that the maximum power point of a solar module (for a given set of environmental conditions) is really dependent on the voltage that it is running at (whereas the current is more constant based on the light level, up to a certain voltage), so to get the maximum power out the resistance of the load needs to be matched to achieve that maximum power voltage (V_MP).

This is what MPPT controllers do, as this maximum power setpoint will change as environmental conditions change.