From version 12c it is already possible to use this feature.
Take a look at this link.
Now, the exact reason for not implementing before, would only be possible to obtain through some note of Oracle itself, but an interesting "justification" addressed by Gary Myers in a similar question is:
It may be just terminology. "AUTOINCREMENT" implies that a record
103 will be created between records 102 and 104. In an environment of
clusters, this is not necessarily the case with sequences. A node can
insert 100, 101, 102 while another node is inserting 110, 111, 112,
so that the records are "out of order". (Of course the term
Quence has the same implication.)
If you choose not to follow the Quence model, then you enter
lock and serialization problems. You force an insert to wait
commit/rollback of another insert before determining which is the
next value, or you accept it, 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 (ie, is
permitted or works as a DEFAULT) or if someone tries
update it. If someone enters 101, the autoincrement "jumps" to 102 or
you risk trying duplicate values.
It may have implications for your IMP utilities and
direct path and retroactive compatibility.
I’m not saying it couldn’t be done. But I suspect in
Finally someone looked at it and decided they could spend time
developing something better elsewhere.
In short, a Quence can do what an autoincrement does and something else, as well as give more freedom to the bank administrator to manipulate these values.
Already possible in Oracle Database 12c Release 1.
– Ismael
So, if you can pass me the source, or some material to read about?
– Luiz Santos
For a long time I have this doubt and never knew exactly why. Thanks that today we can count on the
idendity
.– Ismael
Official link https://docs.oracle.com/database/121/DRDAA/migr_tools_feat.htm#DRDAA109
– Ismael
@Ismael is doubtful what was the advantage they had given up
– Luiz Santos
Hard to know the real reason, probably not implemented because the concept of unique value for Pks and Fks is very abstract, I can make a VALOR_ANTIGO+1 like AUTO_INCREMENT as well as I can generate a UUID that achieve the same result.
– Gabriel Coletta
For me the oracle has always sought to make everything very explicit that is being done, letting you really choose the value of the key and mouth the value in Current for example. In my head was a decision on this line.
– Luiz Santos
Well, the fact is that Oracle did not leave this need without care, it is possible to achieve the same result with a Rigger and a very simple Quence.
– Gabriel Coletta
I think only you. Lawrence Joseph Ellison could answer this, this type of key in general is one last modeling solution when one does not find a natural key , may come from the ancient "implication" of Oracle in relation to this.
– Motta