The oldest rocks on the continents are much older than the oldest rocks on the sea floor. Why?

Subduction is the explanation.  Sea floor keeps being made, so the older stuff has to leave.  It can turn into continent (often does) or it can get shoved back down and replace the stuff that came up from below somewhere else.  Continental material is less dense so will not subduct (get shoved downward) until essentially all of the surface is continent, and even then it probably won't subject, but would instead keep stacking up on the surface, getting thicker and thicker.

Pretty much how ice is "older" than the nearby surface water.

Seafloor spreading  is the cause.  The Mid-Atlantic and Pacific Ridges are pushing the seabed towards the edges of the continents and are subducted, melt and eventually reappear as underseas or land volcanos. The seabed is constantly being recycled at a faster rate than the continental land masses since they are essentially floating on the top of the mantle. You make want to look up photos taken of the underseas "smokers" and volcanic activity on the oceanic ridges.

Simplistically, seafloors are newer.

How do you know that?

Since geologists use circular logic when dating things, the opposite may actually be true