Your misunderstanding here has to do with the way forex works.
You are not buying either currency, you are buying the rights to the difference between the two in U S dollars.
Here’s why they mentioned the two major pairs in a nutshell. If you buy the E/G pair, and either the euro rises in value against the dollar, or the pound drops against the dollar while your trade is open, your trade will be successful. And vice versa for a sell. There is an awful lot more that can happen, but that is the gist of it.
The higher costs mentioned are usually due to the spread being a tad larger, and often the swap rate will be more.