I have yet to ever have to use multiple outbound IPs from the same machine for an HTTP request from Python. It might be "fairly common" for whatever you do, but in terms of the general population, it's not.
Still, it's a fair point that it at least should be possible. It's fine by me if the uncommon 5% tasks are just as difficult to do in requests or even a little more so, so long as the most common 95% of tasks are a breeze.