Because having to reallocate 2 routes every turn isn't fun. And you have absolutely no other competitive choice when trade routes represent 50% of you production, science and gold.
These being so big in comparison to the small yield of these buildings make it so you don,t care about buildings anymore, you just spam them when you cannot make any more settlers or trade routes.
People complaining here don't want a system just like Civ5. They want something fun first and then something that makes some actual sense in the numbers used. Right now you don't build new cities to grab new lands because that place has 1 or 2 more production than that place, no, you make new cities to multiply your trade routes numbers.
now I see where the complaints are coming from, from those who are all about the numbers...
Nobody is forcing you to build cities in bad locations just to get the trade routes... That is your choice to do that, and if it is not fun for you, then don't settle cities in bad locations...
Only settle them in strategic locations, and stop spamming cities, to break the game, and then complain it is over powered..
I find the current 3 routes very fun in both of my games. But I also don't just settle cities 4 hexs away from each other to spam as many cities as possible...
No I don't play for fastest win times. I know some of you enjoy that, and try to find every exploit their is to accomplish that, and with that in mind, you want the programmers to stop you from using those techniques...
the numbers do make sense in a real world setting... large cities have more goods and services shipped in than they produce, and those goods and services do not reduce the food and production available to the cities shipping the goods. If those other cities where not shipping the goods, they would just go to waste.
Small cities also grow via trade, and shipping of goods in. Florida's population would die if it were not for trade. NOrth Dakota would not be growing if it were not for trade.
I know I will loose this fight with all you number crunchers, and it will be nerfed, so that people can finish a game in just a couple of hours, but I will continue to say for me, I like the trade system, as is.