Except back then, you had your app running on the machine all by itself. And there was no need for security -- if putting in a magic input string caused some weird behavior, well, "don't do that". Whereas today that would be considered a security bug (even if your app isn't directly used by non-trusted sources, it could be in a deep toolchain used by some other app).
Not to mention, that when you shipped for an Atari 800, or Vic 20, you only had to test against that -- not an Atari 800 with a Nvidia Graphics chip, vs. a Matrix chip, etc.
What is sad is that the Morris worm dates back to late 1988 when MS was starting the development of OS/2 2.0 and NT OS/2. Yea, I am talking about the decision to use a flat address space (instead of segmented) with default image base addresses which was what made buffer overflow etc exploits really popular.
Not to mention, that when you shipped for an Atari 800, or Vic 20, you only had to test against that -- not an Atari 800 with a Nvidia Graphics chip, vs. a Matrix chip, etc.