You say that but that simply isn't the case. Consider the following:
1. Both parties must agree upon points, and if either player (or both) doesn't have a list already at that level it must be created on the fly.
2. Both parties must agree to allow or not allow Unbound.
3. Then they have to discuss their armies, i.e. tell each other about their respective forces so one side or the other can say whether they think it is hinky.
4. If one side or the other (or both) have Dataslates, Formations, or Books the other doesn't, it often necessitates explaining the special rules.
5. If either side is fielding a Lord of War (or anything huge) you have to see if either side thinks they can handle it.
6. Then there is the chance one side or the other wants you to cut something or says politely they prefer not to play against Knights or something like that.
7. The game might fall apart on ANY of the things above, or you go into negotiation.
8. If both parties are easy going, they might start changing their lists as a result of requests and that takes TIME.
9. Once all parties are content with the opposing lists you still have to generate all the random, variable stuff.
10. And don't forget there is still deployment and terrain setup.
The problem with your "give and take a little" notion is that it assumes that everyone has the ability to quickly alter their lists and just add something. Remember that games played as "pick up game" at an LGS aren't generally arranged in advance. I agree, it should only take a few minutes if either these two things happen:
1. I don't want to play against that.
2. Sure, I'll play against whatever.
*Unfortunately, #1 means no game.. and #2 is becoming rarer and rarer as the balance fades out existence entirely. I miss the days when I could walk into the LGS and say who wants to play 2500 (or whatever level) and in short order someone would be setting up across from me. That was it.