Let’s say you are sitting around a table in a game design studio. You are putting down ideas of what will make your MMO fun and enjoyable. Someone comes up with the idea of rewarding players with killing a said number of some mob, be it spiders, orcs, rats whatever. Everyone agrees, and you start talking about what exact number will be needed to keep the reward challenging, but still fun. Hardcore Billy wants it to be a thousand, and everyone laughs. Kid gloves Joe wants it to be ten, and everyone laughs. Finally the group decides that a hundred is a good number. Some will be killed as players complete quests, to get the player started, others will be killed as they agro the player. The rest will be killed with the intent of getting the reward. After some testing, it is determined that the hundred number works well and is challenging without being too tedious.
I’m guessing that was the general thinking at Turbine when they came up with the kill deeds in LoTRO. Deeds overall are a good idea. It’s not game-breaking, but a nice side bonus for players to collect and customize, adding a much needed layer of depth to an otherwise fairly simply setup.
Why then, do we go from the sane number of around a hundred, to the beyond stupid numbers needed to get the final deeds in the higher zones? Is it more “fun” to kill the same mob in the same zone three times as often as was needed earlier in the game? Or for that matter, is it more “fun” to collect four times as much ore/wood/whatever to advance to the next level of crafting?
Unless you put in three times as many interesting quests to kill those mobs, it’s not fun to sit around, going from spawn camp to spawn camp killing the same mob. What it DOES do, is give reviewers an easy opportunity to bash your game. You might as well put a bullet point on the back of the box stating “Reach level 40 and kill 500 rats, its FUN!”
Keeping the kill number equal across all zones would have been very simple. Find that magic “fun” number of kills, and stick with it. Don’t help reviewers and fans notice you don’t have level 40+ content yet by including such an obvious and stupid time sink. It’s an insult.
Hardcore Casual 
Yeah, the high-level zones in LOTRO have absurd kill numbers for the deeds. I beleive it’s 240 for the title and then another 480 for the trait. It’s too high. But really, those deeds are there for the people who like to grind, and who must complete everything. They’re optional, and what you get for them is really not worth the effort in many cases.
But your comment about “reach level 40 and kill 500 rats” is more or less spot on. That’s why they’re so high at higher levels. There’s thin content right now up there, and so they bet that once you’ve been hooked on this whole deeds system, you’ll spend the time to grind out Determination 7 on those wargs in Angmar, even if you hate it.