Why can't a baby speak

My ditzy side is back, on Monday at my baby / parent singing group I embarrassed myself considerably. At the beginning, of the class the leader goes around the group to get the names of all the children. The baby and I were the first in the circle and the leader asks "so who have we got here then", I wasn't really with it and after waiting about 30 seconds for the baby to say his name I suddenly realised he can't talk as he's only 17 months. After this realisation, I stated "oh he can't say his name sorry, his name is Arlo - sorry forgot he can't speak", lots of laughter and red cheeks at my ditziness followed.  As all 3 sons have been to this group from babies up to school age I was used tp son 1 and son 2 saying their own names it was just natural to wait for son 3 to say his name. Whoops......