To have 100% relative humidity at constant temperature, 3 conditions have to be met:
- There must be water source to saturate space with vapour.
- There must not be present materials absorbing humidity below 100%.
- There must be allowed enough time to reach saturation.
E.g., if there is solid table salt exposed to air, the equilibrium relative humidity would not be higher than about 75%, even if water is there. As salt absorbs water and starts dissolving in it. Either all water evaporates and humidity is still <= 75%, either all salt dissolves and continues absorbing vapour and diluting itself.
But even if all salt dissolves, 100% could be reached only if salt solution absorbs so much vapour we can neglect salt content in the salt solution.
For real life air conditions, varying temperature of free air leads to cycling of evaporation and condensation, resulting in air usually unsaturated. If kept closed at constant temperature, without evaporation or condensation, it stays unsaturated.