In this day and age, there are two good reasons to make your own engine:
#1: If you licence your engine to tons of other studios and it's widely used, so that licencing revenue/revenue cut are bigger than the costs of updating and developping the engine. This is the situation with UE4 and a few others.
#2: If the publisher makes so many games with this engine that it costs less than just licencing another engine. This is the situation with all 1st party engines, Frostbite, etc.
In Luminous' situation, I'd argue it doesn't quite fit with scenario 2, and if you add that it is (most likely) complicated as heck to use, which increases dev time, and as such increases budgets and reduces output, it's just not worth using Luminous. I could see it possibly being used for the rest of the mainline 8th gen FFs, but nothing else.
