Because the
elves and the
Ainur are crazy. All or nothing, yo!
"With but a little of that light, I could totally remake the
trees!"
"Sorry, lady. We've only got a bit of Telperion's light."
"Truly?
Varda never thought to add some variety to the night sky and paste some Laurelin up there?"
"Ah, well, there are some kind of whitish stars, and some kind of reddish stars, and-"
"No Laurelin anywhere? Well, we can't just have one capitalized Tree! That's just plain craziness. Two Trees. That's how it's supposed to be. Wait. How's about
the Silmarils? Both lights there, huh?"
"Sure, let's go ask our good buddy
Feanor to destroy the greatest achievement of elfkind!"
"Hey! You attempting to usurp
Mandos's place as the
god of sarcasm?"
"As a matter of course, no. Either way, you know that he'll make a large deal about how he'll never be so awesome again. Come on. You're the gods! You can entirely whip up some new trees, no problem. Maybe if only one of you could make them, I could understand even one of you not having enough creepy as well as magical power, but it's a team effort, and you guys have plenty of juice to spare."
"Ah, is that what you were thinking? Actually, we're supposed to be hoarding our power, so that when that jerk Mel thinks that he's finally going to be happy, we show up and easily beat him up without even any tension to the fight. Just a quick little tussle, and we'll get rid of him, then go back to hoarding, so that we can do nothing while some famous human does all of the work when Mel comes back for a rematch."
"Oh? Ah, don't you think that my way would be nicer? Additionally, we don't mind a bit of tension, truly. Great stories are full of it! Or we could do away with all of the tension, and you could kick the guy out now?"
"No, I'm sure that this way will work out just fine. Boy, Feanor lives way out in the sticks, doesn't he? Like he's trying to avoid us or something! So impudent!"