It's simple. It's impossible for a turn based combat system like 5e's to be simultaneous no matter how you imagine it. You can imagine it, sure, but you are twisting it into a pretzel that it really isn't in order to imagine it that way.
The reason counterspell can counter a spell is because it's an instantaneous reaction. There isn't enough time in the fiction for the second counterspell to interrupt the first, because the spells are equally long. The first counterspell can stop the fireball since the fireball takes long enough to cast that the reaction can interrupt it and keep it from being successful. By the time the second counterspell is done and can interrupt a spell, the first finished and triggers just before that.
You have to start twisting the fiction where casters are simultaneously faster and slower than each other with the same spell or something equally absurd in the fiction in order to allow one to counter the other. For my current campaign I'm just ignoring the fictional flaw and the spell will be gone in the next one.