Because of cost - particularly given the current state of the RAM market. In order to have so much memory that you never hit memory spikes, you will deliberately need to buy RAM to never be used.

Note that simply buying more RAM than what you expect to use is not going to help. Going back to my post from earlier, I had a laptop with 8GB of RAM at a time where I would usually only need about 2-4GB of RAM for even relatively heavy usage. However, every once in a while, I would run something that would spike memory usage and make the system unresponsive. While I have much more than 8GB nowadays, I'm not convinced that it's enough to have completely outrun the risk of this sort of behaviour re-occuring.

how much swap do you have? i have 16GB now, and 16GB ram. i had a machine before with 48GB ram. obviously having more ram and no swap should perform better than the same amount of memory split into ram and swap.