because vRAM is not the only thing that determines how the card works as a whole. think about a 10 year old CPU paired with 64 GB of ram. would it be faster than a new gen CPU only paired with 16 GB ram??
not a chance!!
now really fast vRAM can actually outperform a lot more slower vRAM so it's not an "always" sort of answer. the HBM used by AMD can do a lot with less due to how fast it is.
in the end it is not just how much RAM, but also how fast it is, the GPU it is paired with and how well the program itself uses that RAM.