Hmmm... The question had me thinking for some time but here is the best logic and reason that I could think of.
I feel the answer to this question lies in knowing why do we need to have a code. I have covered that in the answer to this question.
What is the difference between convention and code?
In brief, a code is brought to
- bring down the size of the convention and
- to make the convention easy to understand and easy to read by giving the reference to the code wherever it is required
Now coming back to the question. Why no code for oil tankers? Let us look for an answer to this why understanding why do we have IBC code and IGC code.
The requirements for chemical tankers are governed by SOLAS Chapter VII and Marpol annex II. Having two similar requirements in two different conventions can lead to confusion.
So instead we have IBC code, reference to which is made in both these convention (SOLAS and MARPOL), wherever required in SOLAS Chapter VII and MARPOL Annex II.
Requirements for the gas tankers are governed by the chapter VII of the SOLAS. SOLAS is a bulky convention and to reduce the size of SOLAS and to make it readable, several codes have been drawn from it.
IGC code is one of these codes.
There is no separate code for oil tankers because unlike chemical tankers, the requirements for oil tankers are not governed by two or more conventions. These are only governed by Annex I of the MARPOL.
And unlike gas tankers, the requirements for the oil tankers are not governed by a bulky convention like SOLAS. These are governed by the MARPOL the size of which is manageable without having a separate code.
If the size of the MARPOL convention increases with new regulations and annexes, the IMO may decide in future to reduce the size of MARPOL convention by introducing new codes. But at this time it is manageable, and for that reason IMO has not introduced a separate code for oil tankers.