Activity Separation field- set an option as default
The change was introduced at https://issues.civicrm.org/jira/browse/CRM-21419, and it gives the reasoning. As with a lot of things probably 50% of people would disagree.

If you want a quick hack (which would get overriden on upgrade though):
Change line 137 in CRM/Activity/Form/Activity.php from

protected $supportsActivitySeparation = TRUE;

to

protected $supportsActivitySeparation = FALSE;