Why isn’t the trade website a feature INSIDE the game?

Simple question. I’m not talking about an auction house. I’m talking about the exact same feature that the current trade website provides.

If GGG is intent on us interacting with other players in game by whispering them partying then teleporting why isn’t all this done via an NPC in game? You’d do the same things, type in your keywords enter your filters then scroll through the items and finally click the button to send the whisper?

Why isn’t that how it works? If anything this is such an obvious slam dunk shut case that I’m actually more curious about how the decision was made to put all that in an external browser?

What’s even weirder is that the real life money store, now THAT is inside the game. But an actual part of the game isn’t included in the game. Wouldn’t it normally be the other way around?

Why?