Hi,
Actually its a good question. I honestly dont know why varchar2 must have its length specified but numbers do not.

Possibly this is why:

CREATE TABLE X(
Y NUMBER,
X VARCHAR2(10))

select * from user_tab_columns where table_name='X';

Notice that the number variable gets a length of 22 (the max size possible).

Whereas I can imagine setting the varchar2 to the max possible (4000)
would waste a lot of space..

I usually just specify a length for both as a general good practice.

BR
Tim