They could, but this is really to our advantage that GUber does not. Think of it this way, everyone on the planet knows you can only fit 5 people in a 4-door sedan. So they order a 4-door sedan and stand there with 5 people on the curb anyway. Adding this "Confirm # Pax" button will not change this behavior.
As the Driver/IC, we control if/how many/which Pins we decide to transport. Be firm, and polite. But be firm. We only pretend to let the Pins think they are actually in control of these decisions. Many drivers make this evaluation curbside...Ultimately, I decide who rides with me not the Pins. Keep doors locked until you have made your decisions.
I may, given observed behavior/attitudes decide to only transport 1,2, or 3 Pins, or to specify which ones I will not transport. For example, the two standing upright can get in. The one sitting in his own vomit can not get in. "But it is your decision Mr. Pins, do you wish to accept this transport under these conditions, or do you prefer to cxl and book other transport. Your preference please?" I also suggest making these decisions based on your safety and profitability. Forget the ratings effects. After a few hundred rides they simply don't matter. And drunks often forget to rate anyway. Focus on making $ safely, not trying to outguess how one trip can affect your silly rating.
And to head off the question, over 2,500 rides in downtown Chicago. Not one email complaint or warning from GUBer on this issue. Zero. 100% success rate with this approach. Your car, your rules. Per our DA, we are only contracted to lift the Account Holders. The driver decides to accommodate, or not, their special requests such as additional pax/cargo etc. I know, right? Empowering.