Whenever a site collection is created using PowerShell, the default security groups (visitors, owners and members) are not provisioned.
When site collection is created via browser an additional method call is made to create the security groups which is not called while using PowerShell.
The following scripts will help create the default groups.
Call method CreateWebDefaultGroup passing the parameters context(clientcontext) and UniquePermissions(bool)
CreateWebDefaultGroup -Context $ctx -UniquePermissions $false