The Mac is a "cooperative multi-tasking environment".  What that means
is that ALL applications must cooperate to allow each other to run.
That implies that when an application is in the background it should
not do anything that would interfere with the smooth (and usually
interactive) nature of the foreground application.

Each application must periodically call WaitNextEvent in order for
other applications to run.  If an application executes code that
waits for user input, and during that waiting WaitNextEvent isn't
called, all other processes will not get any CPU time.

A solution?

There is a shareware (or freeware) system extension called 'okeydokey'
that will automatically select the default button after a preset time
interval.  You can find the init in the 'cdev' (control panel device)
section of Macintosh archives (like sumex-aim or Umich).  I've never
used it.  Good luck.