It's linguistics.

Of course you can open a raw file, as the term "to open a file" has a very broad spectrum of different, but related meanings.

• On a low level, "to open" just means to read the bytes in the file, and leave it to some software to do with them whatever the software wants to do. You can do this with any file in any format.
• On a higher level, "to open" means to open and "understand" the bytes to an extent that a useful, intellegible representation can be shown to an end user (e.g. the text from a Word document shown in the correct fonts and sizes). This needs software that knows how to interpret the bytes.

A raw file contains the data as they were read out of the sensor, in a vendor-specific format, in theory without any further processing. So:

• On a low level, "opening" a raw file gives you the intensity-per-sensor-pixel values, which is not very useful to end users.
• On a higher level, "opening" a raw file includes converting the low-level data to an RGB image that can be viewed on screen, or saved in e.g. JPEG etc. But converting from raw to RGB needs a lot of image processing and non-obvious parameters (white balance, sharpening, gamma curves etc.). So, when you "see" a raw file on screen, the result is influenced by many (arbitrary?) settings that have been applied in the process.

Whenever you "open" a raw file, you do not really see the true contents of the file, but rather one of many possible processing-based interpretations of the raw file. You can legitimately use the term "to open" for that, but need to be aware of the processing that has been done in between.