What about projectile travel speed? And lag period after a successful attack? You don't want to just right click, you need to cancel your animation at the soonest possible moment so you don't get counter harassed. It's more difficult for melee since they obviously have to get up to the creep to do it. Then there's varying amounts of damage, how targets might shift, and how good your opponents are at it as well. And you have to keep watching your mini map as well as opportunities to harass/dive for first blood at the same time.
No really you guys are talking from the perspective of someone who's done this for a long time, at least I hope you are. It is by no means an easy skill to pick up.
You deny their creep and so you get a reward out of it, just like killing a creep does. Killing anything gives you a reward, why shouldn't denying do the same thing?
Well, I'm not sure if I ever said it shouldn't give you an award, since denying has to have ramifications to mean anything, but Riot decided their game would be better without it and I could understand their position.
But if you wanted to talk about the psychology of the matter then you can distinguish between last hitting a creep and denying like this:
When you last hit a creep you're in control of what happens. The consequences of getting the hit on an enemy is obvious.
But when someone denies you a creep it's different. Suddenly you have to take into account their skill level respective to yours. And the effects of denial are more subtle. A new player who keeps getting denied would not feel the impact of his mistakes until about level 6-7, when everyone has their ults except the new player and he's suddenly outpaced. At that point it's too late to correct it, so clearly they realize they clearly made a mistake somewhere. However, it's not immediately obvious where they went wrong, which leads to frustration and a less enjoyable match overall. Multiple instances of feeling "left in the dark" concerning the games mechanics will ultimately cause them to abandon the game, feeling like it's too hostile and unfriendly. Which is absolutely true. Dota is not very forgiving and you really have to want to get better if you're going to play it for a long amount of time. The metagame's changed since 6.1x (which is when I started) and you're pretty much expected to know how to deny even in APEM games, where newbies, originally, congregated before they joined TDA or some other tryhard clan/community.