Leandra, websites can (and do) have different styles for different devices. This can be implemented for PCs and not for smaller devices, say. Or it could be an option that you choose in the settings. If you're on a PC (using Chrome), try my extension (there are links to it in the answer I gave (to myself(?)) below). See what you think. If you don't like it, you can uninstall the extension. If you find the extension useful for its main purpose (making wikitables from Ancestry/Family Search pages) but hate the vertical lists, let me know and I'll think about making that an optional feature.
Having said that... I don't think a quick flick of the finger on a touch screen device is really such a problem. Also, WikiTree isn't just about individuals - it's about the connections between them. A vertical list of siblings with the subject of the profile placed into them gives you more context (at a glance) of that person's life. You can see, for example, that they were the first child, who traditionally, in many cultures, has had the most responsibility of all the siblings. There have been thousands of studies looking into the effects of birth order position, and they tend to show similar features. The horizontal lists without the subject in them loses that insight. Not every person conforms to the birth order 'stereotypes' but I think they are helpful to understand a person.