Most modern movies are shot in Panavision or similar, with an aspect ratio of between 2.25 and 2.4 to 1. In order to display these without cropping the sides, it's necessary to have the black bars top and bottom.

Similarly, if you watch an old film shot in 4:3 (the standard up to 1955 and for many films after) it will be shown with black bars at the side to preserve the correct appearance. The smaller number of films shot in VistaVision are usually displayed in 1.85:1 with very small bars top and bottom. Only films specifically made for television are shot in 16:9.
Some channels - such as ITV and Channel 5 in the UK - zoom Panavision films up to fill the screen, thereby losing some of the picture on each side: evidently Netflix does as well (though many of their films are likely to be made for TV). The BBC and Channel Four do it correctly, as does Sky and the Apple TV (and The Amazon Fire TV for that matter).
If you are really determined to lose the bars, at the cost of also losing some of the picture, many TVs have a 'zoom' facility which will fill the screen.