I think people saying this are meaning a different type of "hard" than you are meaning.

Relationships require effort. But in my experience a healthy relationship doesn't feel "hard" in the sense of miserable soul sucking exhausting effort that other "hard" activities can be.

People say "hard" in contexts like "a hard life", "hardship", "hard times" etc to indicate something that was exhausting, draining, depressing and full of suffering.

In that sense, only a bad relationship feels like that.

But I would say a healthy relationship requires the same kind of effort as maintaining a garden does, consistent awareness and tending but while some activities can be exhausting (like healing childhood wounds, learning to communicate in effective ways when it's not something that comes naturally) the majority of such "work" does not feel like that.

It's the difference between dragging yourself to work for a job you hate, vs engaging in something meaningful and overall enjoyable that still requires effort.