What is it, really, that makes them so different other than how it's consumed?
Well, a lot. I think
@Story_Marc talked about it briefly. Though he is usually busy, you can try asking him about this topic.
Since he is going to be busy, I guess I have to mention a couple things. First of all, formatting is different. A lot of people read web novels on their phones, some even write web novels on phones. Compare a phone with a book. Even small books are larger than phones, plus, the books page won't go anywhere even if you trace it with your finger. This is the first difference.
The second is difference in publishing methods, expectations and overall culture I guess. Like, even if your book is a freaking kickass story that is really fresh, unique and interesting, you probably will be left unnoticed if it's finished and has like 60-100 thousand words. Reason for that is, people who read web novels read different stuff, and expect different stuff. Obviously not all of them, but there is a reason why SH is smuthub, RR is LitRPG fanatics, and so on.
There are also multiple differences in how you write stuff. Like, when you write WNs, you can have less details, faster pace, less descriptions, more telling rather than showing, and so on. Partially because you compensate for this with the amount of chapters. Like, if you have 300+ chapters, more than million words in a single book, obviously you have to write it differently. And I won't even talk about chinese web novels that have 1000+ chapters. I hope I'm not too far off from the truth and it will help you at least a little bit.