Because it doesn't even begin to address visual and auditory range. Take these two scenarios.
1. A group of orcs is standing around in the dark in an enormous cavern debating the perpetual issue of whether hobgoblin or dwarf tastes worse. A party of torch-wielding adventurers traipses into the cavern at about 1500' distance. The orcs clearly see the adventurers' torches, because they are the only lights in a field of black. The orcs should be well aware of the adventurers before they get anywhere near them. They should have no chance of being surprised under normal conditions. And since they know exactly where the adventurers are, they can choose to initiate an encounter by making long ranged attacks at whatever distance they want. The adventurers won't even get a chance to make any checks unless they are close enough.
So, assuming the orcs decide to hide when they seed the torches, what determines when the adventurers are allowed to make Wisdom (Perception) checks? If they have to wait until they approach within 1d20 + 20 feet, (or even 1d20 + 20 x10 most of the time), the orcs can still choose exactly when the encounter begins by launching their attacks before then.
By the book, it would seem to require a 1d20 +20 x5 or x10 (depending on how the DM interprets the cavern's terrain) encounter distance roll...but the adventurers would be able to make Wisdom (Perception) checks at that time, and the orcs would still individually need to make readiness checks to avoid being surprised (unless one of them was keeping watch).
2. Same scenario as above, except that there are no lights. Orcs have 60' darkvision, adventurers don't. Everyone is out of sight, at least until 60', but nobody is hiding. Do we just roll the same encounter distance and then start? Do we drop the x10 modifier and just use the dungeons version since it's purely auditory--even though we aren't told to do that?
By the book, no Wisdom (Perception) checks are made or required (since no one is hiding--everyone is simply out of sight). You roll encounter distance, then everyone makes their check to see if they are surprised or not, then encounter begins. DM: "You see and hear nothing. I'll draw the location of orcs on your map. Roll initiative."
The point is that they need to include better rules for how sight, hearing (or other) senses on one or both sides interact with the otherwise excellent surprise and encounter rules. The concern I have is that they will under-address it if they read something like this, by simply saying, "If one side has the opportunity to sense the other side first, they can begin the encounter whenever they wish, and the other group is automatically surprised." They would create additional problems and still leave ambiguities if it were addressed that way, however. It needs some careful consideration.