It's still a trek across an ill-developed desert frontier to actually get to those rivers, a frontier that's been penetrated by the Turks and Persians multiple times in the past. The comparison between Mesopotamia and the colonies does not fly, because Mesopotamia is directly adjacent to hostile Ottoman lands, unlike the overseas colonies which are separated by sea and surrounded by swarms of angry Roman ships. The African ones aren't even held by Romans, they're held by clients that pay trinket tributes.As to "too distant", this is the 1600s, the Romans have territories (admittedly very autonomous) in the East Indies, and North Africa. Mesopotamia is a trip down the Euphrates. Make sure that is secure, and the Tigris is the fall-back frontier for Mesopotamia.
EDIT: Religion is also against the Romans here. Unlike the OTL Ottomans, the Romans are most definitely not Muslims, and the Muslims ITTL follow an ultraconservative doctrine borne out of hatred of Christians. There's no feasible way for the Empire to get the Turks to cooperate with them, and the Kurds aren't numerous enough to make up for the lack of support.