Because the "Room" is often the biggest problem in getting quality recordings. Most rooms are not acoustically treated. So a omnidirectional microphone will be very susceptible to unwanted reflections, room resonances & other issues. Even in professional studios that are treated, I often use isolation panels with cardioid mics to further isolate. Room acoustics are almost always the #1 problem or #1 benefit to quality recordings.I don't quite get why salespeople recommend cardioid mics so much for simple recording use. A good multipattern mic would suite most users better.
Get the full room sound and pick up the whole band with an Omnidirectional mic.
Keep in mind louder sounds mask quieter ones. Players closest to the mic will be louder in the mic.
Space out accordingly.