Dependency on php5.6-simplexml goes unchecked, is that a bug?
This is a good question!

Until very recently, this issue hadn't come up, because just about every distro packages the XML extension with PHP.  Since you're using Ubuntu 16.04 with PHP 5.6, my guess is you're using the Ondrej PPA.  For whatever reason, he packages the XML extension separately.  And of course very few people used this PPA with CiviCRM until recently, but now people want Ubuntu LTS with PHP 5.6.

So yes - in theory this check should happen at install.  In practice, it doesn't really come up except for folks using the Ondrej PPA (and, I suppose, people who compile their own binaries).  So I'd guess this would be considered a "patch welcome" situation - now that PHP7 compatibility has landed in CiviCRM, my guess is that this will bite very few people (who will probably find this page).

If you feel strongly about this and are motivated to submit a patch, feel free to hit me up on chat.civicrm.org (@junglebird) if you need help getting a CiviCRM dev environment together.