I don't think alcohol tastes bitter, if you drink straight vodka it doesn't taste like much of anything it just burns going down. I think alcohol makes drinks more bitter owing to the fact that it's a pretty good solvent that can extract bitter compounds from the drink's other ingredients so you taste them more easily.

Also a lot of beer is flavored with hops (tho most beers don't include as much of them as your typical IPA) which is typically bitter. I've had tea and sparkling water that were non-alcoholic but had hops as a flavoring component and they were unpleasantly bitter to me.