Solo sex is... different. As you said, you can have any partner(s) you choose. (You listening, Jason Momoa?) Your imagination is the camera operator, seeing the whole scene, a close-up of what's most interesting at the moment, things you cannot really view while having sex with a partner. And the lighting is better than in reality even with skilled people setting it up; parts of our anatomy are in creases and shadows. The "you" that you view is often more beautiful than in reality, as well as younger, better groomed, slimmer, and all that.
With a partner, you are rightfully concerned about their pleasure as much as your own. Solo, though, it's all about you all the time. What do you want, right this moment? Got it. It's not even being selfish!
Although there can still be a wet spot, if you're a woman.
And last, as a happy couple ages, there are physical limitations, even impossibilities, that may make sex more difficult, painful, or just not as fun. For me, that's not a license to find a new, more able partner but to arrange for some alone time.