It's because author of the game has decided to link the plugins into the executable, and they are always extracted to that storage location and used from there no matter if the same plugins would be available globally.

I don't know why Hollywood can't write into the softlinked directory... could it be related that directory creating doesn't seem to work from the MorphOS shell into softlinks either?

System:> makelink softlink t:
System:> makedir softlink/testdir
Cannot create directory softlink/testdir
object is soft link

Is this a bug somewhere?

Other kind of file operations seem to work:
System:> echo "test" > softlink/testfile
System:> copy MorphOS.readme softlink/