"The DEFAULT value clause in a data type specification indicates a
default value for a column. With one exception, the default value must
be a constant; it cannot be a function or an expression. This means,
for example, that you cannot set the default for a date column to be
the value of a function such as NOW() or CURRENT_DATE."
Yani, bu explicitely sınırlama belgelenmiştir
Eğer TRIGGER
oluşturmak varsa MySQL Sürüm < 5.6.5
BUT
MySQL 5.6.5 changelog stats
a>
As of MySQL 5.6.5, TIMESTAMP and DATETIME columns can be automatically
initializated and updated to the current date and time (that is, the
current timestamp). Before 5.6.5, this is true only for TIMESTAMP, and
for at most one TIMESTAMP column per table.
Referans: