Why?
Actual studies that look at evidence and test hypotheses seem to suggest that mRNA vaccines are effective.
The vaccine efficacy (how well it teaches the body) and the body's own anti-body efficacy (how effective the anti-bodies are against the virus) also have a role to play.
Thus, a vaccinated person can still be infected, and can even spread the virus around. The sooner the virus is dealt with within their body, the sooner they no longer have a virus to spread. Isolation helps sooner, too. It is therefore possible for a triple vaccinated person to still get infected and to test positive for the infection.
Among various benefits of being vaccinated are the lowered load on hospital staff. The number of hospital beds and even oxygen cylinders does not matter as much as the availability of hospital staff. This pandemic helped highlight that around the world, there are already only a limited number of doctors and nurses, and that both of them are already over-worked. For e.g., I did not know that 30-hour shifts are considered normal. It got me thinking that I wouldn't want to risk receiving care from an over-worked and tired medical care giver whose judgement might have been impaired from sheer exhaustion.
Many of us vaccinated people are now able to safely isolate at home when we are infected with the Covid virus. We rush to hospitals only when the anti-bodies that we create are unable to deal with the Covid 19 virus. Over-worked hospital staff thus focus on a far lesser number of patients. There are now more beds available for those who need them. These included those vaccinated but ill enough to require external medical care, not-yet vaccinated, vaccine-hesitant, those who can't be vaccinated, those who won't be vaccinated.
Really, this pandemic won't truly end until we isolate and end the spread of the virus. Being vaccinated planet-wide and reducing the load on the hospital is our next best bet to ensure that those who need the medical care have less-overworked staff and beds and oxygen cylinders.
The various successful track record of anti-vaxx conspiracies, the true stories of the greed of medical companies/hospitals, the CIA messing with the polio vaccine in Pakistan + Afghanistan, the might-is-right approach by which some countries grab vaccines and waste them while other countries do not receive even a single dose, the hiding of data to save "face" and reputation, the political attacks based on data, politicians speaking up instead of scientists, scientists not knowing how to speak to media, media irresponsibly reporting news - all of these contribute to a general decline in the acceptance of vaccines.
In my lifetime, polio was almost-eradicated. Smallpox was eradicated even earlier. I am hopeful that in time, we'll all pull up our socks and eradicate Covid-19 as well.