Microsoft Outlook by default uses a semicolon to separate multiple recipients. You can change this to a comma, but why is the semicolon the default?
Microsoft Outlook was originally positioned as a business product, and many businesses complained that the use of a comma as a separator created havoc because they have a policy of setting names in the address book as "Last, First".
In 2000, the Outlook folks tried to change the default, but the outcry from corporations made them go back to having the semicolon be the default separator.
Besides, there are a lot of people who have commas in their names, such as Martin Luther King, Jr.