Part of it goes in the main story, just the play-through, getting all items, side quests, that sort of things. I always thoroughly go through everything and that already covers a lot.
Training Pokémon and building a team also takes a lot of time, especially if you're EV training and IV breeding, also if you're breeding for the right nature - I have spent an enormous amount of time trying to get the right natures for my Pokémon. And if you haven't already focussed on catching all Pokémon you encounter, after the Elite Four, most people go through the whole map again and catch all Pokémon they missed before, evolving Pokémon and obtaining the right items to do so (evolution stones, hold items, etc). Probably also a lot of trading since some Pokémon are version exclusives.
When you've done all this thoroughly, with maybe some lazy moments and just walking and exploring, got the national dex and so on, I can imagine if you have a lot of playtime. Of course not 600, but imagine you got the game as soon as it got in store, and have been playing it ever since, and using it as your main game, for trading and battling online, that probably covers it, as years pass. There are also moments where you just leave it on standby to get a drink or something, all of these moments add up.
I never had anything close to 600, but I'm planning to use my current game as my main one, I have somewhere around 100 right now and I'm not even close to the end, so yeah. I hope this helps out a little, otherwise I don't know how people can get 600+ XD