Imagine someone from an old aristocratic family that has a noble title, was known to own land, etc. Even if they lose their money for some reasons they won’t be considered in the same social class as someone who comes from a poor background.

You can do the opposite, imagine someone from a poor neighborhood who then start to gain wealth, their money alone will not make them be part of the upper class (im quite sure there is a woody Allen movie with this exact premise).

What distinguishes social classes is in part wealth, but that’s just one component.