Because they hang around too many fools and or drop their guards about wearing masks. Still, all it takes is one quick second to be infected.

There is no vaccine for Omicron. Like the Flu vax's, historic strains covered not new ones.
When the number of breakthrough infections is greater than 50% (vaxxed can catch and transmit still), you have to question why do we still refer to these medicines as vaccines?

Vaccines are not this "magic shield" that will always protect a subject from infection. Vaccines are never 100%. Some do come damned close, but none are 100 %. Put simply, a vaccine instructs a host's immune system to treat a pathogen as something to be killed ASAP. This leads to one thing: lower count of that particle. For viruses that is call a "viral load". A lower viral load does 2 things: decreases the number of viral particles that are able to cause the disease, and decreases the number of particles that are released by the host. The reduced number of particles in the host will either prevent the disease from happening, or reduce the symptoms of the disease.

If a virus reproduces faster than the immune system can fight, then you can get a breakthrough infection. However, due to lower viral load, you won't experience the disease nearly as severe as if you had not been immunized. In addition, the likelihood of you spreading the virus to others is reduced, because while you may be feeling symptoms, you are not expelling as many particles. If you are not giving off as many particles, someone else may not be exposed with a sufficient number of particles that their immune system will not be able to kill off the virus before infection.

Because the vaccine doesn't stop COVID completely, but it will keep the vast majority of people out of the hospital and alive. It's like the flu vaccine, only probably better.