What's the relationship between effort and zazen?

I'm currently reading Zen Mind Beginners Mind which I am enjoying. However when I read Zen books I'm always confused about the role of effort in zazen practice. Reading the book it makes it seem like there is no effort involved. In one sense I can subscribe to this. However if zazen was truly effortless then everyone would be doing zazen most of the time. That seems like a flawed conclusion to me. Can anyone help me with how effort relates to zazen please.