Most of that is probably just done in post, if they have any type of real setup. They would usually have the guitar mic on one channel, and the voice mic on another. Then in post they just cut out the channels that aren't being used. There isn't much of an excuse these days, as pretty much any type of recording setup with mics would allow two channels.
If they are using a cell phone, or mics on the camcorder, well those often are just not good microphones. The band I was in used to try to record rehearsals with a vocal microphone into some big zoom recording thing. When I brought my little zoom H2 with mics actually designed to capture sound coming from all directions, the results were infinitely better. If I was actually motivated, I would make more sound demos with it. Alas, I am not.