Well, even that's apples and oranges. Serenity was a cheap-as-hell movie ($17 million from memory I think?) as far as big special-effects films. The amount of people campaigning for Firefly after the show was ditched was still pretty laughably small, they were just very vocal. And half of that was Joss fighting so hard himself to get it made. It's not like Universal spent a significant amount of cash or layed out much risk to resurrect it, Serenity was pure fan-pandering (as much as I'm a FF/Serenity geek and I'm glad it got made).
Animorphs is a little different. To do book #1 with any kind of production values you're still looking at, say, $80-90 million at the least, probably more, higher-end blockbusters are like an average of $150-ish.
And unless they're ultra-confident they're making that money back, no studio's going to greenlight it. That means a PG-13 rating (which, sure, you don't want it R anyway, Animorphs has still gotta be viewable by older kids, say 10-11+), and that means playing their game since you're spending a buttload of their money. Likely means making it much more "good v.s. bad", softening a lot of the terrible decisions the kids have to make, and even simple stuff like "you're showing grizzly bears and tigers fighting with teeth & claws, but no blood's allowed on screen". You don't want anything gratuitous, no, but you do want to keep that ability to suspend disbelief, it's gotta have a little grit and realism. Stuff like The Hunger games has such heavy themes, but you don't see any exploration of it on-screen and it all feels really sanitized and safe. I wouldn't want that for Animorphs. You'd have to go, like, Terminator 2 in tone, without all the cussing, pretty much. PG-13, but right up there bordering the line and pushing the limits of that. It'd have to be a little more visceral than, say, The Avengers or the recent Batman flicks, to keep any sense of it not being laughable.
And that's not gonna fly these days, you'd have the choice of "soften it up", "suffer an R rating and we'll only give you $20 mil to make it, and the kids all have to be approaching 18", or "be satisfied with the books, don't mess with a good thing by trying to force it into a movie series". You just can't adapt/translate the books as they are and keep any sort of reasonable budget for effects and production, the reality is you'd be playing ball with the executives and getting something palatable, that they're comfortable selling Happy Meals with.
It's been that way since Batman Returns in '92, unfortunately, and even that wasn't dealing with kid protagonists, or close-quarters fighting with tooth & nail. Animorphs has the whole...genocide & slavery thing going on, to boot.