I went for an outing and 3 of my friends who had covid and are fully vaccinated got it a second time. why would this happen if infection develops antibodies and so does the vaccine ?

Vaccines are designed to prevent illness, not infection per se. Being vaccinated protects a person from serious illness and hospitalization.

Like influenzal viruses, COVID-19 is a mutating virus which can come in multiple variants, and some vaccines cannot cover the newer mutants well. So, depends upon multiple complex factors, but the three major vaccines, Pfizer, Moderna, J&J, provide 94% relative protection but not 100%.

The illness and vaccines though instigate antibody production, however the virus is able to change such that the produced antibodies may not work well but could prevent severe disease. You get influenzae or common cold regardless of having had it before because of their similar abilities to change and evade the body immunity. It is important to wear mask and continue hand hygiene.