The majority of Televisions and Monitors back then were all 4:3, and there was no standard for communicating between device and display the screen format. Yeah there was a few high-end sets that used a wide format, but mostly they catered for letter-boxed movies by cropping the video. So mostly to avoid confusion of settings and distorted picture, most games did not offer any support for a widescreen display. Surprisingly though, 007 Goldeneye (a launch title) [Edit: it's about a year after launch] had a variety of screen settings, full-screen, letter-boxed wide, letter-boxed cinema and lastly an actual 16:9 setting. This doesn't mean the Nintendo 64 had native support, Goldeneye did this software-side, as well as the other games that opted to have an option.