I'm sure some people are phonies. On the other hand, I think some people honestly like to just play with a baby for hours and hours.
Think of it as a form of peaceful meditation. It's like playing with a dog or a cat. The creature isn't going to yell at you, or judge you, or make fun of you. They will smile, and laugh, and give you 100% happy feedback for as long as you tickle their toes or such. That can be very satisfying if your real life is stressful and full of troubles.
On one hand I normally like my time spent more "engaged". I would rather read a good book than play with a dog or cat for hours on end, with just tickle - smile - tickle - smile. But on the other hand, if I was feeling stressed, and I really needed to just not think about anything at all and let the world go for a while, that is probably just about the perfect activity. You're getting positive feedback, you're engaged with another living creature, and you're focused on something so your other stresses are temporarily gone.
It's why all those studies show that having pets is such a powerful force in reducing stress

In a way, a baby is like a pet that doesn't stay in that docile pet-stage for long

Not that I'm advocating having a baby so you can have a pet. But I'm saying that the baby-playing can have benefits for some and that they can honestly enjoy doing it.