Lots of great posts above. For me it's:
Nostalgic reasons. Late 70s early 80s were my formative years and a lot of that time was spent in arcades.
I love the feel of pinball. There aren't many things that feel better than a perfectly flipped ramp shot, or a spinner into a loop, or a scoop/ball lock.
The challenge. I played a lot of sports as a kid through early adulthood. The last time I played basketball I tore my meniscus in a father/son basketball game in February. Pinball is a much safer activity to get the competetive drive going.
Social interaction. I love having friends and family over to hang out and play. There's nothing better than watching a little kid standing at one of your machines wide eyed and grinning ear to ear.