The "why" is easy.
These US vaccines, both Moderna and Pfizer, are not like any vaccine, and instead of containing inert pathogens, have mRNA instructions for our ribosomes in our own cells.
These mRNA instructions seep into our own cells, get read by our ribosomes in our cells, and then our own cells start growing spike proteins.
That is dangerous for many reasons, such as these damaged cells that are sprouting spike proteins will be attacked and destroyed then by our immune system, and there is no control over how many cells will get affected or where. The mRNA instruction are injected into the arm muscle, but can accidentally travel anywhere, and if they get to someplace delicate, like the heart or brain, you die.
But the fundamental flaw with mRNA is that since it contains no inert pathogen, it is trying instead to get the immune system to remember and trigger on spike proteins alone, and nothing else.
That is bad for 2 reasons. One is that it can't possibly work and the immune system can't remember spike proteins, and two is that if it did work, then the immune system would attack the spike proteins on our own exosomes, and we would then die.