So the question: Why have the high neckline?
The bra needs to encapsulate the breast, which means it must support underneath to hold things up where they should be, but they also need to restrain the top so they don't pop up uncontrollably with each bounce. The motion control neckline exists so that the entire encapsulation of the breast can occur. After all, it doesn't do any good to have a well supported breast underneath if you still get a lot of bouncing on the top.
A solution some have found helpful? The ENELL Lite bra. The neckline is lower than the Sport bra because it's designed for lower impact activities such as yoga, golf, or walking. For activities where the support doesn't need to be as iron-clad, the Lite version is a great alternative.
Hope this helps explain the reason behind the neckline - it's one of the things that makes the bra stop the bounce. What other questions can we answer for you?