When determining the default size of the local cluster, MATLAB uses the number of true cores you have (hyperthreading is ignored). You can modify the size of the local cluster yourself.
1. On the Home tab, open the Parallel menu.
2. Choose "Manage Cluster Profiles...".
3. Select the local Cluster Profile and click the Edit button. At that point you can set the NumWorkers field to whatever you like (8 or even more).
It is generally best practice to have the same number of workers as true cores on your machine. Opening more workers than cores doesn't usually improve performance.