5 Queues Don't Queue Versus 5 Queues

This is happening on a consistent basis and I actually don't understand the logic behind it.

I duo queue with a friend in flex queue and a large majority of the games I am put against a team of 5. I do not think this should ever happen. With the sympathy of riot you mostly get put with a 3 queue if you duo queue and are against a five queue but what does this do. A 5 queue will almost 70% of the time always beat a team that doesn't have a five queue.

Why would it not be that a team of 5 is placed against a team of 5 all the time?

Is it for queue times being too long? When I queue as a team of five in flex it is ALWAYS one sided. The queue times are also ALWAYS 1-5 seconds long. I do not think that making 5s play vs 5s would make the queue times long enough to have this addressed.

What is the reason? Thoughts? Why is it like this? It inbalances the entire flex queue system.