Basic defense posture doesn't include the ability to leave the five-foot square you're occupying at will, which is what one would need to do to avoid a person walking into their square with their arms wide open.
Think about it this way. Look around the room you're in. Assuming it's not unusually large, there is no way an invisible creature could avoid you for very long if you were to put your arms out to your sides and walk quickly around the room, so long as you didn't move in any kind of predictable pattern.
Put me in an average-sized living room and I could easily find an invisible creature, even if it was specifically trying to avoid me. It wouldn't even take me very long to do it. And so could you. I guarantee it. Now, most dungeon rooms are somewhat bigger than that - but with a party, you also have more people searching.
Think about it this way. Look around the room you're in. Assuming it's not unusually large, there is no way an invisible creature could avoid you for very long if you were to put your arms out to your sides and walk quickly around the room, so long as you didn't move in any kind of predictable pattern.
Put me in an average-sized living room and I could easily find an invisible creature, even if it was specifically trying to avoid me. It wouldn't even take me very long to do it. And so could you. I guarantee it. Now, most dungeon rooms are somewhat bigger than that - but with a party, you also have more people searching.