my uneducated take is maybe other civilizations existed on earth or other planets but then self-destructed or got destroyed by a asteroid just like the dinos.

my uneducated take is maybe other civilizations existed on earth or other planets but then self-destructed or got destroyed by a asteroid just like the dinos. if human life on earth is a nano second in a day of the planet's life. surely these things can come and go without leaving any traces. poof just varnish into thin air.