Because LLMs are so unreliable. Alexa misinterpreting and giving a nonsensical/unrelated answer is annoying, but fine. If it was an LLM, when it was wrong it would be lying confidently and unless you're double checking everything it outputs (in which case just search it that way in the first place) then that means it'll be misinforming you without you having a clue, as opposed to the current system where it's obvious when it gets it wrong. That's clearly just worse.
And that's ignoring the fact that all the publicly accessible LLMs have been "jailbroken" and you can get them to say all manner of wild things, and Amazon is definitely aware that all it takes is one or two viral clips of Alexa saying something unhinged for their sales to start dropping, and the tech isn't currently sufficiently understood for them to avoid that.