It’s not even true (anymore) that CURI is only listed on NASDAQ. It’s traded at lots of venues but there is at least a benchmark bid/ask based on US securities law.

In fx it’s the law of whoever has the most information because there is no benchmark market at all. It’s why fx pricing is so hard to do right.

Based on our experience speaking with a number of institutional players, including traders and banks, what we found is that the institutional rates are unbelievably efficient, but the situation immediately gets worse as you move away from these centers of liquidity.

If you talk to 3rd party institutional FX providers, they'll immediately mark up rates because its how they make all of their money.

What you say doesn't really apply to currencies, at least not to major ones.

All currency exchanges track each other extremely closely, because it's extremely easy to arbitrage between them.