Quite simply, because there's not a thing we can do to prevent it.  The processing power that would be required, for an eReading device to see and recognize that a text-box was going to break across a screen, and fix/adjust everything, on the fly, is simply too much to be available inside a simple, dedicated eReader.  The software and firmware is coming along, so that someday, those "if/when" scenarios might be manageable, but for now, they're not.

There's also a school of thought that having the boxes break across screens creates an overall look that is less choppy than constantly having large clumps of white space at the bottom of pages, because the text boxes won't fit.  We don't have a strong feeling about it either way, but as there isn't much that can be done about it...there isn't anything to choose, at this point in time.  

Or, Why is the Bottom Line of my Box/Top Line of my Box not Showing?

Typically, it's because it's nearly hidden, at the top of the following page.  Try changing your font size, either up or down, and you'll likely find your MIA bottom line.