We all do it at least sometimes, either out of laziness or bad habit (or more specifically, lack of a good habit). The opponent attacks, and the ball is perhaps foot or two away, or maybe just a few inches, and rather than step to the ball, allowing us to use our honed blocking technique, we instead just reach for the ball, and improvise the blocking stroke. You can get away with it to an extent, but the cost is an erratic block.
In the large majority of cases, it’s not that the player isn’t quick enough to take that step; it’s a matter of not having the habit. It doesn’t matter if the ball is one foot away or one inch, the stroke should start with your stepping into position so you can have a repeatable stroke, rather than an improvised, awkward one. Even against an extremely hard-hit shot you should reflexively be stepping toward the ball even as you reach for it.
You shouldn’t think of it in terms of whether or not you have to step to the ball. You should assume you have to step to the ball, and be flexing your knees slightly as the opponent is hitting as you prep yourself to move. And then, 99% of the time, you step to the ball, even if it’s just a one-inch step. To develop that habit takes practice, but with practice, it becomes a habit. Once it becomes a habit, you’ll have a much better block!