The inactivated polio vaccine, for example, does not prevent infection. It just prevents disease. That means that it cannot confer herd immunity (vaccinated people can act as asymptomatic carriers, leaving unvaccinated people at risk), but for most people who are vaccinated, it eliminates the most serious consequence of polio infection - paralysis. The SARS-CoV-2 vaccines might end up being the same, conferring protection from disease but not from infection, and even that would be important.
Without a placebo group, how will you measure effectiveness and safety? You need to monitor a statistically equivalent group of people to determine the difference in rates of infection and disease, and to look for side-effects.
> Would the researchers not want people to go out and live their normal lives?
That would be great for the trials (assuming both the vaccinated and placebo groups behaved identically). But most people would consider it unethical for researchers to encourage the trial participants to increase their likelihood of exposure to the virus. People who sign up for vaccine trials are not signing up to get CoVID-19.
It’s spreading like wildfire to people following best practices too all over the world. It only takes a small subset not following these practices to spread it all over.