Yep. The beer you buy at the store was packaged in an aseptic and oxygen purged manner, as it is intended to sit around. Your local brewery sunk serious money into a bottling or canning line capable of doing that. A beer poured at a taproom, whether into a glass or any other container, is just a beer poured. It is instantly exposed to the atmosphere and everything in it. If it sits around, it's no different than any glass of beer you pour sitting around. It will quickly go stale due to oxygen exposure, and then eventually crap will start growing in it.
(This does illustrate why many homebrewers' kegging or bottling practices need revision. If you are not packaging under CO2 counterpressure to a liquid purged vessel, don't be surprised when your beer doesn't taste the same when you serve it as it did sampled from the fermenter. If you are packaging for a competition -- a timely subject -- remember that the judges will be getting your beer in, potentially, the same condition as the OP's disappointing sample.)