Uncontrollable variables are not really constants because when collecting data, you must allow it to stay random.
For example, if you were running a sample survey to find out what people's hair color was - then hair color would be an uncontrollable variable because there would be many different possibilities, not one closed set or constant.
And so that's really why uncontrollable variables are not considered constants.
In order for a variables to be considered a constants it would need to remain unchanged throughout the experiment.
The control variable would also need to have a direct impact on the values; as in it would be a constant factor in all data in relation to variables that are open to change.