Why can't "[[:space:]]" match Line Feed(LF) on grep command?

Issue

• Why can't "[[:space:]]" match Line Feed(LF) on grep command?

  # cat /etc/hosts
  # Do not remove the following line, or various programs
  # that require network functionality will fail.
  127.0.0.1               localhost.localdomain localhost
  ::1             localhost6.localdomain6 localhost6
  
  # grep "[[:space:]]localhost" /etc/hosts 
  
  127.0.0.1              localhost.localdomain localhost
  ::1            localhost6.localdomain6 localhost6
  # grep "[[:space:]]localhost[[:space:]]" /etc/hosts
  #
  

• I expect the following. Because "[[:space:]]" includes LF.

  # grep "[[:space:]]localhost[[:space:]]" /etc/hosts
  127.0.0.1               localhost.localdomain localhost