Because the roleclaim itself is information, and not claiming gives scum an even better smokescreen than vanilla claims, since it leaves the door open for them to claim a real role at their convenience. At LYLO, the game of keep-the-roles-alive-and-don't-tell-scum is *over and done*, even if it's a situation like this game where there's potentially three days of LYLO left. If town votes wrong in LYLO, the game is over. Period. Long term role plans are all but worthless, the thing that matters is lynching correctly in the short term and any information that can potentially help in that regard is worthwhile and necessary to bring out.
"If I'm the Doc and we actually manage to lynch scum that day instead of a townie, my neck is as good as chopped from the scum for my declaration. But if I don't declare, then what are people to think? 'I MUST BE SCUM!' seems to be the order of the day in that scenario. So, I can be Honest Townie and claim Doc (and risk losing Town one of it's best assets), or be Bad Lying Townie and claim vanilla and hope it doesn't come back to bite me (which I think I can agree is always bad juju to lie)."
If you are Doc in LYLO your night action is no longer a valuable asset to the town, because if town does not survive the day it becomes irrelevant. Your claim of Doc can be valuable, since by matching it to your behavior town can make a better decision about who to lynch that day.
"I don't see the point of telling the scum who every single town role is, since they just have to claim vanilla or something unprovable and the town has no way of telling who's telling the truth, while the scum get what little info they didn't already have when they've already got the upper hand. The full roleclaim outs the doc that might save and therefore clear someone, the watcher that could see something important, or other roles that the scum don't want to have around. Meanwhile, the town gets... a bunch of information they can't confirm and nearly half of which is false. I really don't see why the trade is worth it for the town."
The town gets information, some truthful, some false, and can work to separate the false from the true and therefore make a more informed lynch decision and have a better chance of continuing the game. By not claiming you're risking the entire game, betting that town will lynch correctly today with incomplete information against your night action being more useful. THAT is not a good trade to make for town.
By LYLO, the scum probably already know who's got a role if they're halfway competent. Even if everyone left is vanilla and claims such, you're not at any disadvantage compared to not claiming, and if there ARE roles left, precludes a scum pulling a last minute "Oh I'm actually X" claim out of nowhere to save themselves, by forcing them to declare it and defend it up front.
Edit for QR's last post:
Stop relying on roles to prove truthfulness and look at behavior. Don't say that it's worthless to claim when your claim can't be confirmed by a role. It can be confirmed by looking at your behavior. If anything, this is more reliable, since who's to say the guy claiming watcher isn't lying too?