Your assumption is that the state of a dired buffer is trivial to recreate so should be thrown away. While that is the case in the simplest use case of using dired as a replacement of a bog-standard file browser, it is possible for the state of the dired buffer to be more complicated. For example, the buffer could contain marks and flags for some batch operation the user was working on before they decided to visit a subdirectory. The dired buffer may not even correspond to just one directory but may be a result of a complex find operation or may contain some arbitrary set of directories the user manually curated. It would certainly not be nice to the user to lose all this state accidentally.

This is why emacs takes the conservative approach and chooses not to throw away dired buffers by default and gives the users the option of choosing to do discard the current buffer consciously by using the "find-find-alternate" command (bound to "a" by default in dired-mode-map).