Webnovels are mainly focused on reader self-insert wish fulfillment. Communities tend to become more homogenized. Take those two together and you'll see that reading sites will eventually become homogenized around certain wish-fulfilment genres.
A good example of this is Inkkit and Royalroadl (there was an 'L' there before at the end). A long time ago, these two sites started at around the same time. They have varied stories. I used to put my first webnovel on both sites. Eventually, Inkkit became more like Wattpad (Romance self-insert wish fulfilment), while RR shifted more to OP MC. This is also due to a conscious decision of the RR Admin. They did this by intentionally not using specific romance genres (like specific pairing tags, etc.) so that RR will not become wattpad. There were ancient debates back then about it, but RR admin shot it down.
As a side note, romance wish fulfillment is huge, and dwarfs all other types of self insert. If RR did not discourage, RR would've been full of romance novels. So, if not romance, then RR went the OP MC route of wish-fulfillment.
In the beginning, RR's OP MC was just fantasy setting. It eventually became more specific to litrpg as years passed. Also probably due to RR starting out as the translation site of Moonlight Sculptor. Wuxiaworld on the other hand started translating Coiling Dragon iirc. Not sure. SH became smutlandia because it's very loose here with the rules, these started as the refuge of those not allowed in RR.
TLDR, webnovels are (generally) going to be reader self-insert. The wish-fulfillment fantasy specific to each site came to be because of their own histories.