Casual walking, especially on flat ground, isn’t challenging enough to get the Zone 2 heart and health benefits. It isn’t enough to get and keep the heart rate at 60-70% of its max. For most people.
To get around it, you could find a place with more hills. But that’s not always convenient. Especially if you live in the Netherlands. Or in any of the other flat countries I can’t think of right now.
The easiest workaround is to put on a light backpack.
Sure, you could also just pick up the pace.
Personally, I rather wear a backpack so I don’t have to obsess over the pace.