...well, yeah. There are three Sinnoh titles still readily available for purchase. If you expected the majority of those critters to be found in HG/SS, that's on you; logic would dictate, accurately, that there's going to be a significant divide so as to give you every opportunity to dig into the Kanto and Johto Pokémon and to give you incentive to buy a Sinnoh title if you've not done so already.
This one's also on you. No one forced to to devour the whole thing in the span of a few days, and 20 hours is about the standard length of a game from Start to "Finish".
The joy of a Pokémon core title, of course, is that the "Finish" is anything but. If you're going to consider the simple obtaining of the badges as the end-all, be-all of the game and ignore the countless side-quests and other stuff to do before, after and during the main quest and basically ignore a huge part of the game, then yeah, you're out of stuff to do.
So much of this thread is people complaining about the most minute and meaningless elements, stuff that there isn't actually a problem with, just people complaining that the game wasn't tailored to their every specification at every turn.