As of version 12c you can already use this feature.
Take a look at this link .
Now, the exact reason for not implementing before, would only be possible to get through some note from Oracle itself, but an interesting "justification" addressed by Gary Myers in a similar question is:
It may just be terminology. "AUTOINCREMENT" implies that a
103 will be created between registers 102 and 104. In an environment of
clusters, this is not necessarily the case for sequences. A node can
insert 100, 101, 102 while another node is inserting 110, 111, 112,
so the records are "out of order". (Of course the term
sequence has the same implication.)
If you choose not to follow the sequence template, then you enter
blocking and serialization problems. You force an insert to wait
the commit / rollback of another insert before determining which is the
next value, or you accept this, if the transaction fails, you will have
a space between the keys.
Then there is the question of what you do if someone wants to insert a
row in the table with a specific value for that field (that is, it is
allowed or acts as a DEFAULT) or if someone tries
update it. If someone enters 101, autoincrement "jumps" to 102 or
you risk attempting duplicate values.
It may have implications for your IMP utilities and
direct path and backward compatibility.
I'm not saying it could not be done. But I suspect that in the
Someone looked at this and decided that they could spend time
developing something better elsewhere.
In short, a sequence can do what an autoincrement does and something else, and give the bank administrator more freedom to manipulate those values.
