It is entirely possible (and if you knew the reason for it, you would probably think it's the smartest approach to use)
To render/display a PPT file you need to fully understand every nuance and detail of the PPT file format - and you must also display everything (transitions, graphics, timings, backgrounds, fonts) exactly the same way powerpoint does it. (no point converting if the output looks nothing like the input).
Now to do this your program basically has to display everything and work the same way that powerpoint does - even for features that maybe are quirky, strange or rarely used. Features with their own "features" (or bugs) must behave and display the same way in your conversion.
If Microsoft makes a change between versions of Powerpoint - your conversion software output must still look the same as the original viewed in Powerpoint.
Now if you are not a software developer this might not seem like a big deal... until you realise that what is required... you need to also develop your own powerpoint, and keep it in sync with Microsofts' version.
The alternative - it to use the routines and code inside Microsoft powerpoint, and code to it's own documented interface. So you are basically having your program "drive" or "control" powerpoint to handle the render of it's own presentations. If Microsoft change the way PPT works - no worries, because you are using their PPT code. The "update" is automatic (most of the time - except when they change their external interface).
So what happens when you only have powerpoint viewer. The code you expect and need to be able to hook into (the full retail powerpoint program) just isn't there, and the program doesn't run.
What happens when you have Open Office? Same deal. The Microsoft Powerpoint libraries are not present, so the converter doesn't run.
The point that you can convert and load Open Office powerpoint document files in Microsoft Powerpoint isn't the issue. The converter relies on the Microsoft code libraries to decode and display a PPT file.
That's just how it works.