Turn off pivot deletion when soft deleting in the main table

Turn off pivot deletion when soft deleting in the main table

I have a Users and a Groups table. There can be multiple users in a group
and a user can be in multiple groups, so I use a pivot table to realize
the many-to-many relationship. In real, the relationship isn't
many-to-many because every user can be in only one group, but the system
was designed this way. I soft delete the rows in the Users table so if
needed I can restore users later.
The problem is that when I delete a user, the system automatically delete
the entry in the pivot table too. This is always one entry. I didn't set
it to behave like this, only added the protected $softDelete = true; line
to the Users model, so I don't understand why the system automatically
delete the pivot entries.
I don't want to soft delete the pivot entries, I want to soft delete only
and exclusively the users, and the system shouldn't touch anything else.
I could create my own delete function that set the deleted_at variable to
the actual time but this way I couldn't simply turn of the soft delete by
changing the true to false if I need this.
Why does the system automatically delete the pivot entries, and how can I
turn off this behavior?