I see several tools on the pages I cited that are a few tens of dollars.

Just because YOU think Outlook should be able to do something doesn't mean
everyone believes that, but let's assume that you're correct.  How would you
combine the information?  Would a contact item whose creation date is later
than another's creation date take precedence?  Would you want ALL of the
data in the newer item to overwrite ALL of the data in the older item, or
would you want only selected fields to overwrite?  Perhaps you think only
empty fields in the first item should be filled in from data in the second.
If so, what do you do with the data in the fields that aren't empty?  Should
an item you're importing always overwrite the item already in the Contacts
folder?  What if the item in the Contacts folder is newer because you've
added it by hand before you decided to import older data?  What if there are
some newer and some older contacts in the imported data?  Would you want to
always overwrite the existing contacts when importing or ignore those
contacts in the imported data that already have entries in the Contacts
folder?  What if the contact in the folder and the contact in the imported
data are identical in every respect except for the business phone number?
How would Outlook know which to use?  If Outlook were programmed to combine
items the way YOU want them combined, how does that affect someone else who
wants them combined differently?  With all these decisions to make in the
importing process, the program that implements them would be fairly
substantial.  I'd rather not have Outlook burdened with that much extra
code.  It's complex enough as it is.  I'd rather have a separate program
that I can purchase or not.