I don't see why. At the end of the day (partitioning SNAFUs aside) nobody cares if the angry birds binary lives in /usr/bin, somewhere under /opt or anywhere else, since all user interaction with it is just launching it and that's done via its .desktop file. On the other hand I really wouldn't like having, say, an /opt/openssh/usr/bin/ssh that looks for its known_hosts file in ~/.config/ssh/, an
/opt/rsync/usr/bin/rsync that calls that and both present in my $PATH and being search every time I type a command. My mileage obviously varies but I like to think of my boxen as integrated systems rather than receptacles for independent self-contained apps.

Strictly speaking perhaps not, but if every external lib has to go to its own special directory under /opt then packaging anything that uses those is going to be a pain in the neck and the library search path is going to be long and slow to traverse. I guess it's much easier to sweep these problems under the carpet by making a policy that forbids external dependencies.