You are correct, the ideal omni mic picks up sound equally for 360 degrees around it. However, practically that never happens, due to a multitude of factors, among them being the diaphragm diameter, the way the capsule is mounted, the windscreen, and the size and geometry of the mic body. The human ear is essentially an omni-directional pressure transducer. You can hear sounds all around you, but due primarily to the ear canal and outer ear, you hear things, particularly highs, much better when they are in front of you. An omni mic is no different, it has a preferred axis, usually perpendicular to the front of the capsule.
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