Why? There's no connection between the planet a trade good is built on and your revenue from trade. Trade goods aren't really even traded, unless you choose to do so in the diplomacy screen.
Sorry it took so long to get back CornhuskerMac. Let me explain myself. I try to focus planets for specific jobs (tech, shipyard, economy, political and trade). I don't like to waste spaces by planting a trade good on a tech world, once they're there you can't move them. I to try place my trade planets in a remote corner of my empire to get the most money for route length. I balance that with planet quality; however it wouldn't make sense in some situations. By sending all the freighters from those two planets, I reap the following benefits:
1) All the trade routes converge, taking fewer ships to defend and generating similar predictable revenues.
2) Placing one star base to cover both planets will also cover all my routes.
3) I can hit each foreign planet only twice, avoiding trade deficits. Trade deficits are bad, the AI doesn't like it.
Trade is more economical this way. It would not be as effective to send freighters from 9 of my worlds to Altaria.
Economy drives the military. By keeping the all the eggs in a hard to reach basket I create two very low maintenance worlds (check how much it costs to maintain a trade good) generating revenue to fund quite a few worlds, which will be very hard to take and hold (unless they're my last two). An enemy won't destroy the colony and they can't demolish the trade goods to make it more defensible.
And even though production on these worlds STINKS (I only put one factory, which eventually goes away), after it takes off, I have that couple of 10,000 BC to outright buy the new trade goods as I develop them.
And that's my reasoning. It's just a plan to lower the expense of maintaining a strong economy.