Our employee notices are sent home through the mail. It is just a postcard, that if you are planning on attending, you need to return to HR.
I work at a large hospital ( ~3500 employees), and with the varied shifts, vacations, many people do not have e-mail access (i.e. the housecleaners, cafeteria staff, patient transporters etc.) so they want to make sure they get the info to all employees.
It is not possible to include the information with our paystubs/direct deposit notices, because they get printed out on a certain type of form that becomes a self-sealed envelope thingy.
My DH works at a very small company, about 11 employees, so they just send an e-mail with the date, and as the picnic gets closer, they send the details/map etc.