The sparse maps in these open-world games are actually a strength, not a problem.
There are many different kinds of open-world games, and each creator has their own ideas about what makes games fun and interesting. One difference between open-world games is not only how big their map is, but also how full it is of places, people, and, most importantly, things to do.
Many games try to find a mix where the map is interesting but not too full, but some open-world games have maps that are purposely sparse or even empty for a number of reasons. For example, games that focus on survival and exploring the natural world tend to have more empty maps. Similarly, historical games that are all about realism often feel more real when the maps are sparse and set in the countryside.