Love reading posts like these. Sure, a lot of the Gyms are super easy, but a lot of them, particularly in Gen 4, are actually hard. Let's look at Sinnoh, specifically Platinum:
Roark: Headbutt off Attack equal to Gyrados (125) isn't fun so early in the game, plus flinch chance. Not hard, but nowhere near Brock easy (tho harder than Roxanne too)
Gardenia: Not THAT bad (I have literally soloed with a Budew before, or at least beat her Roserade with it). Still though, Reflect up with Roserade out is kinda ew.
Fantina: This is just awful. Makes your shiny new EQ TM useless, and Mismagius's 105 Special Attack Shadow Ball should not be a thing so early on in the game. While there are a few counters (Luxio/Luxray, Floatzel with Crunch, Vespiquen with Toxic) and you can kinda brute force it, it hits so many things super-effectively. Duskull with Will-o-Wisp also screws over a few of those.
Maylene: A breather boss to an extent. Her mons can take a hit but there are loads more counters to her than the last couple of Gyms.
Wake: Odds are if you go in blind you will get wrecked. Waterfall Gyrados? Ice Fang Floatzel? I mean Quagsire is easy but it has good type synergy and all. Bottom line, really good team, and Waterfall Gyrados is enough of a challenge alone.
Byron: LOL, nice break, though not braindead easy. Every starter kinda laughs at him (tho Torterra has to watch out for Steelix Ice Fang and Empoleon has to watch out for Magneton).
Candice: Surprisingly tough. Snow Cloak Double Team Froslass is scary, and the rest of her team has good coverage.
Volkner: Not too bad, mostly because most of his team is easily walled by any decent Ground type (watch out for Luxray Ice Fang though)
All of the E4 are rough because their coverage is insane. Galactic is also a threat early on with Purugly and Skunktank, and Cyrus 3 is incredibly hard. In general, the game is one of the hardest games, but for the right reasons (unlike HGSS).