Seriously, expecting anyone to know all roles is kind of ludicrous. It took me a really long while to not suck as a single role, and I can't fathom the amount of time it'd take me to get good at a second role, let alone all four. It's not like back when I had lots of spare time to practice playing Smash and Team Fortress 2 anymore...
It's an interesting question. On the one hand, I agree, it's crazy, and Riot agrees too since they designed the new champ select to ensure you only ever have to play two roles. Even pro players didn't actually play every role well -- they'd have like one Challenger role, one maybe Master role, two high Diamond roles, and then like a Gold role that they would just dodge. (One aspect of League I personally hate is that effective dodging is an important skill when it comes to maintaining a high ranking.)
On the other hand, like, the barrier to entry to Ranked is already ridiculous. If you just grind out SR games and don't monetize, you need to play something like 300 hours of League of Legends to get access to ranked. That's just getting to level 30! You probably won't actually have enough IP to purchase the 15 champs and the tier 3 runes you need at that point. Expecting you to play every role a little bit is almost not that unreasonable if you've put that much time in. People often have much higher expectations. (If you actually go to Reddit, one common piece of advice is to just play only normals and keep going to op.gg until you start seeing gold or platinum players in your normals, and THEN go to ranked. So literally their expectation is that you will already be better than 50% of everybody who plays ranked before you start doing it.)
I'm not sure the expectation people should have of League (or the expectation Riot has) is that Ranked is the natural play pattern that you should be heading towards as quickly as you can. There are a lot of players who never play against other humans, much less play in the most competitive mode the game has. It might be better if they framed Ranked as a pursuit for people who were extremely driven rather than as the "normal" way to play League.
But that would probably cut into their KPIs, so.