Because 12 is also 0 and therefore the start of the time convention. On the 24 hour clock 12am is written 00.00 and 12pm is written 12.00
Just the same with birthdays, we are born at zero and celebrate our first birthday at one, we do not start at one.
AM and PM are abbreviations for ante and post meridiem. Meridiem being the middle of the day when the sun is at its peak so AM is the 12 hour period before that and PM is the 12 hour period after that, ante (before) and post (after) being from the latin language that is often used for scientific words and expressions.
then why are you not 0 yrs old when you are born?
ok, who writes "goo goo at a at a, fukin jew heeb"?
"fuckyoubigot i dont like you're sig" "repeated annoying comments" The Lord is not my Shepherd for I am not a sheep.