For an actual literal switch, there are several reasons to not include one. For example, there are issues of long term reliability. The MFLB needs to switch fairly high current (more than 5 amps) AND any switch used must be very small. This means that "a simple switch that is reliable" is also more expensive than you might expect (think many dollars, not pennies). There are actually very few switches which would even work in the Box, despite the variety of components available these days. (Small means low-current; high-current means large—realistically, there is not much overlap).
Attempting to work around the limitations of available switches unfortunately also requires too many changes to the design. Using high energy electronic methods to get around the problems has significant impacts on the overall reliability as well. Instead of trying this approach, we have instead introduced a "battery pushback ring" that can act in the same way as a push-button switch. For units so equipped, the back end of the battery becomes the push-button -- pressing the battery into the unit turns it on, and releasing pressure will turn the unit back off. These pushback rings can be also added (retrofitted) into Launch Boxes shipped without them—see the Online Store for details.