You can use OpenCL just fine on Nvidia, but CUDA is just a superior compute programming model overall (both in features and design.) Pretty much every vendor offers something superior to OpenCL (HIP, OneAPI, etc), because it simply isn't very nice to use.

I suppose that's about right. The implementors are busy building on a path to profit and much less concerned about any sort-of lock-in or open standards--that comes much later in the cycle.

OpenCL is fine on Nvidia Hardware. Of course it's a second class citizen next to CUDA, but then again everything is a second class citizen on AMD hardware.