Hi, someone who doesn't do third party publishing currently, but has worked alongside of a lot of pubs/devs that do. Personal thoughts, not Riot's!
Many, but not all, revshare agreements between a publisher and developer have the split based on how much equity & resources the dev requires, and what the publisher believes the opportunity cost affords in ROI. A 50% split+ usually (but again, not always) means there's conditions like a dev studio requiring additional advanced funding to get the title made, depending on where in development & the scope of the title, in which the publisher takes on more risk in investing. In these cases, the risk is mitigated by the publisher by a higher revshare %. In cases where the developer doesn't need advances on revenue, to my knowledge it's pretty rare for the publisher cut on revenue to be astronomical unless the developer isn't (or doesn't know how to) shopping around for publishers.
I really want to stress that "the publisher's capital equity investment" part, because that's really how many big and mid-tier publishers view the risk/reward on ROI. As long as I can remember, investors and publishers generally have the upper hand in this negotiations not out of cruelty or greed necessarily (although there definitely are publishers that are absurdly greedy IMO), but because there are a billion aspiring & proven dev studios & teams that have a billion great game ideas, and if one studio has significant capital requirements to get it's game into the market & wants only the publishers to take 30%, well, there's like hundreds of other studios that probably need less capital and would *kill* for a 50/50. There's a lot of nuance in that rev share split though: one big example is when you're an established leader or team of developers that is setting out on your own, or a mod team that is getting into making your first big game, or even a completely unknown development team that also happens to have a KILLER prototype. There's a lot of nuance, and as to what is a right rev split really is complicated IMO, but much like all modern day businesses, whoever controls the operating capital generally has the most favored terms because of their "risk".
Additionally (but again, super not always!), the games publisher represents the actual work of publishing, which can represent actual cost centers the dev might not be able to do. This can include various degrees of marketing, distribution (ESPECIALLY working with first party & digital platforms, in many cases, and even more so as asia & mobile become more of a thing in the west), certification, localization resources, customer support, legal (this includes trademark, copyright & IP enforcement), and even brand equity (i.e "if you get published by us, that grants you a lot more credibility than other publishers"). This is where a lot of weird math can come into play on revshare (or even IP control), but the tldr in my personal experience is "you get what you pay for, as long as you do diligence on the publisher's efforts".
Finally, and it's kind of an aside, but publishers can help significantly with how the title is distributed & "placed" on platforms (think consoles, mobile stores, Steam/Humble/EGS, etc), but with rare exception it's not common for the distributor to publisher/developer to be significantly changed, unless we're talking AAA schanigans or platform exclusivity terms. It's easier than ever for a studio to get their game onto most playable platforms all by themselves (assuming they have the capital, can deal with cert & the other platform requirements!), so most of the publisher's work focuses on the distribution terms: stuff like when and how long a title is promoted on certain areas of a digital store, or if the distributor will pay for an exclusive trailer, or better sales terms, etc etc.
Since I'm not actively involved in third party publishing, not too much I could share in the actual sausage making, or even deep thoughts on what a right or wrong split looks like (although I mostly advocate developers having the most favorable terms possible). Hope this adds some context!