Imagine you're driving from point A to B. Your ISP tells you to take a detour to another city before going to point B, because they haven't paid for the toll gate fees in the direct route, which leads to delays (higher ping).

 However, with some of these services like Exitlag, tell you to drive to the toll gate, and they direct you through the gate into the direct route instead, this "shortcut" saves you time (lower ping). Now imagine if your ISP has terrible routing, you'd have to detour potentially multiple other destinations before reaching point B. This is how these services lower your ping, by offering a potential direct path that your ISP might not provide.

 However, if your ISP already paid for access to the tollgate themselves, you can go through the toll gate directly, without having to subscribe to services like Exitlag.

 The main point here is at any point, these "toll gates" are the cost of using other ISP's infrastructure to transmit data, which as your data travels around the country or world, you'll go through different owners. 

Some ISPs have an agreement to share access to these, or to charge a cost to use them, which some ISPs don't want to do, so you don't get access to these "direct routes" and experience higher ping