It is a fact that real-world omnidirectional microphones are not equally sensitive in all directions.
But first, why would you buy an omnidirectional microphone anyway?
Has it ever crossed your mind that you really need a microphone that is equally sensitive in all directions?
I would imagine not. Mostly we use directional microphones because we want them to capture the sound source, and not other sounds from around the room.
And even if you did want to capture sounds from all around the room equally, the result would be a tremendous confusion.
You can of course use omnidirectional mics intentionally as ambience mics. But you will find that cardioids work just as well in that application (there are reasons why you would probably not choose figure-of-eights).
In fact, with cardioids you can point them away from the direct sound source and get a greater proportion of reverberation, which you can mix in with your close mics.
No, the reason why people sometimes choose an omnidirectional microphone is that omnis have great clarity. The polar pattern just happens to be omni – it's the clarity they want.
The reason for this is because omni mics are simple in design. Figure-of-eight mics are simple too. Cardioid, hypercardioid and multi-pattern mics are complex, so you don't expect the sound quality to be quite as natural.
The plain fact about so-called omnidirectional mics is that the polar pattern generally narrows at high frequencies. So you still do consciously have to point them at the sound source.
But this doesn't really make much difference in practice. If someone came out with a genuinely omnidirectional microphone, it wouldn't shatter the Earth; it would be nice, but wouldn't make all that much difference.
Isn't it nice when there's something you don't really have to worry about?