Why? Let's say I have a transaction spending 0.1 BTC, paying 0.066 to one address and 0.033 to another address, and 0.001 BTC in fees. I bump it by including a second input of 0.001 BTC, all of which goes to the fee. The two outputs remain unchanged.
Having an unused output of exactly the amount you'd contribute to the fee is quite unrealistic IMO. At least not if you consolidate and/or mix UTXO's regularly (which you should).
So you'd have to add an UTXO which is larger, either resulting in a higher change output or creating a 3rd output which is then guaranteed to be the change.
But the option should be available for those who understand the risks.
I personally would like to have that feature.
However, lots of people would use RBF because they believe they know how to handle that while in fact they don't. These people will inevitably compromise their privacy.
And i believe that was the reason to not include RBF.
And let's be honest.. if you know what you are doing, you rarely ever need to use RBF anyway.
I can't remember the last time i had to bump a fee.