... and most often isn't, except when it is! Just to expand our view a little:
In the last few years, I've been playing the '30s-era "Smile" with my local library mandolin group ("Smile though your heart is aching, Smile even though it's breaking ..."), where I alternate between guitar and bass guitar. While the sheet music, in F, specifies an F-major chord over the first several lines, the MELODY itself (whether instrumental or sung) follows the "descending bass line" that's been discussed above, whereby each of the first three lines starts on F, E, and D. This effectively sounds the chords of F, Fmajor7, and F6, before diving into a somewhat jazzier finish of the progression.
To confuse the issue, what I just called F6 shows in the sheet music as the "slash" chord of F/A (just a form of Fmajor, NOT an F6), which should be confusing as it's under a melody note of D. BUT, that chordally-consonant A in the bass provides a real nice set-up for the following Abdim chord, followed by Gminor ... yet more descending bass line!
When I realized what was going on here, that opened the door to all sorts of fun, especially when playing bass! Mostly, I hope my comments provide some context for the seemingly-varied opinions above.
BTW, that, IMHO, lovely lead line & chord progression was written by (wait for it!) ... Charlie Chaplin!