Because numbers, without context, are nothing but numbers.
It's why champions with high skillcaps have low winrates in solo queue, but are often picked in LCS. No, they are not weak. You get a high winrate or a high skillcap, rarely if ever both.
As for Zilean, the best thing I can gather is that his sample size is pretty small which can inflate his winrates (only mains playing him mid, people picking him only as a niche pick in the right comps, etc).
Looking at other stats, he deals below average damage compared to other midlaners (again, context) and incredibly below average kills. From what I gather here, it seems he's effective for his utility (he gets lots of assist, deals below average damage, doesn't get a lot of kills) rather than his damage.
I would like to know why he has such a high winrate, when people say that his damage is more unreliable than before.
See, this is the problem with your statement. You look at one stat (winrate), and bring up one aspect (damage). Nothing else. If you read between the lines, it's obvious Zilean is played mid as a utility mage with some poke to aid his team rather than solo carry. The guy has his faults. People talking about his damage aren't wrong, again he doesn't deal as much damage and gets a low number of kills in comparison to other midlaners.