Any particular reason how this evolved? And opening scenes in general for all types of TV shows? And why a tag scene for sitcoms but not dramas?

More commercials of course.

Some old dramas, like the Quinn/Martin productions, had tag scenes they called Epilogues.

The FBI, Act I, II, III, IV, Epilogue.

For sitcoms opening scenes and epilogues provide prime editing-out material for syndication.