Why does Pages have such great publishing capabilities compared to Keynote?
because Pages is a dedicated professional text application with many page layout tools.
force text to dynamically wrap around images. Pages can do it, but Keynote cannot. Why not? It would be very helpful. Or does it? Does anyone know?
Because Keynote is a dedicated presentation application to display text and images for presentations and does not have advanced page layout tools that a page layout application has. Screen based presentations by there nature require a much more simpler design and therefore require simpler tools.
A workaround could be to convert the text box to shape, via Format > Shape > Make Editable to turn the box into an editable shape, then double click the box converted into shape to edit its paths, position your mouse over an edge while holding the ALT key to add path points and bezier curves. This technique allows you to reshape the text box into wathever you want, conforming other objects and simulating text wrap.