The answer (which you're probably not going to like) is that WikiTree is a wiki. Wikis are supposed to be easier for people to deal with than actually learning to code HTML. (I learned HTML first, and have been working in raw HTML for so long that I actually find wikis harder to deal with than straight HTML would be, but that's because what most people mean by "easy to use" is actually "what I already know". Probably normal people would look at straight HTML that I code without even thinking about it with horror, and scream, "Aaaaah! Give me back my wiki!")
But quite aside from discussions about which is easier to learn or use, both wikis and HTML require putting in the time to learn how they work. (Personally, I have fairly standard wording that I use when I'm adding a birth, marriage, census, or death record, so I just copy it in and then edit it with the data for the record I'm actually looking it.)
But the point, as Philip pointed out, is that WikiTree uses a language. (In this case, mediawiki, which is the form of wiki that Wikipedia uses, and probably the most widely used wiki there is. [There are other wikis with wildly varying syntaces. That's one of the things that drove me nuts when I first started working with wikis: no two are exactly alike. At least HTML has standards to follow.]) It may not be intuitive, but those who have taken the trouble to learn it know how it works, and if WikiTree were to make unilateral changes in the wiki we use here, then people who know how to edit Wikipedia would not be able to transfer their knowledge of wikimedia here, nor would WikiTreers be able to transfer their knowledge of how WikiTree works to Wikipedia. (Actually, WikiTreers would have to throw out what we've already learned about wikimedia, because we'd be using something different: "bride of wikimedia" or "son of wikimedia" or some kind of sequel-ish name like that.)
More critically, the changes that you're asking for (boxes to fill in) aren't the find of thing that any wiki can do, because the point of a wiki is that people don't have to drop into the "source code" to see what's going on. There are lots of HTML editors which look WYSIWYG on the surface, but then have a "Source" button that lets you see and fix things when the WYSIWYG editor messes up the HTML [which isn't uncommon], or when you want to do something more complicated than the WYSIWYG editor can handle [which also isn't uncommon]. So far, wiki advocates have claimed that wikis are simple enough that people don't need that kind of functionality.
Personally, I disagree. I think it's long past time that somebody put out a wiki editor which would let people use a WYSIWYG editor (like we have here in G2G), including things like filling in boxes for adding sources, and then have the editor generate the actual wiki text (or "source code"). As long as there's a button which lets those who have bothered to learn mediawiki syntax drop down to that level and work in it, then in my opinion, the site would still count as a wiki, but it would be a lot easier for people to learn to use.
P.S. If you want Chris to see and comment on this thread, I recommend that you add "Tech" as a tag. I don't know what other tags he follows, but I know he follows that one.