I am going to flip that question on its head a bit. What is the incentive NOT to make your own blocks? If you already run a node that has its own mempool, why would you want to be beholden to someone else dictating what your miner is hashing? What do you get out of that that you would not get if you created your own block templates?

In my mind, the only reason not to create your own block templates is if you aren't running a node. And then question is, why the heck aren't you running a node?

And then, yes. OCEAN gives a discount on the pool fee for those who use DATUM to create their own templates, and you pay zero pool fee whatsoever if you are lottery mining using DATUM. So the incentive is you get paid more in either scenario. Assuming you are fortunate enough to get paid at all in the lottery mining option.