Quoted from TopMoose:

It's a dopamine rush - your brain is rewarding you the same way it does when you get a Facebook notification, hear a great song or taste your favorite food. With pinball, you're getting chemical rewards whenever you complete an achievement, beat a timer or get it to make a certain sound or display a special light show or animation. Dopamine can be addictive.

Quoted from jackd104:

To build on this, I’ve often reflected how pinball relies on certain mental abilities that were very important in our evolutionary past, but not so much in the modern world. Namely, mental geometry in calculating angles and using precise timing to hit targets, catch something, or prevent something from escaping. Think how close this resembles skills needed to hunt (hit a moving target by chucking a rock) or more advanced methods like bow and arrow or later guns. Or Spearing a fish in a stream. Or brains reward us for exercising skills important to our survival. So our brains like it when we play pinball. This is only one dimension of the answer and many others responses only build on this.

Quoted from wayout440:

I don't need to play it to get a rush. It's like any other hobby you find enjoyable. I like bringing the past back to life and fixing broken things. I could just as easily be into something like restoring old arcade machines, old vending machines, jukeboxes, or automobiles. Any of those are useful items that you can do things with, just like a pinball machine. Hobbies that are static such as collecting stamps or most other "just look at it and that is all you can do with it" are not for me.

Quoted from Luckydogg420:

Let me ask you, have you been playing video games for years before finding pinball?
I think the the lure of pinball has something to do with the fact that it’s real. The tactile feedback is unmatched. You can see, hear and feel the ball rolling across the playfield. Pinball gives back in a way that video games just can’t match.
And then, you can take the glass off and explore the world in a box with your fingers. You can touch it, clean it, take it apart, fix it, put it back together better then it was before, and then to top it all off, you can personalize the machine to make it yours.
Video games, board games, physical sports, none of them have the same level of involvement as pinballs do.