he isnt necessarily. in melee, it was really really close between the two. i think fox was considered slightly better because other high tier characters had an easier time egdeguarding him, and you even had peach that went even with him. with fox, he is harder to edge guard and will live longer since his recovery is better in every single way. the fact that fox is also the overall quickest character in the game, has more reliable offstage edge guarding abilites (shine spike), and has slightly more kill options than falco seems to sway the public in his favor (fox's main killers are upsmash, uair, shine spike, and bair; falcos are dair, fsmash, bair, and uair. for falco, his fsmash and uair dont always kill as reliably as fox's upsmash and uair do).
in PM however, you could actually easily argue that falco is better than fox. for one, fox was nerfed a bit more than falco was, although slightly indirectly. the damage change to distant lasers means that fox's laser camping damage racking abilities were hurt a decent amount. for falco, this isnt exactly as bad since his lasers are more used for set ups as opposed to damage racking in comparison to fox. the loss of invincibility on shine hurts fox more also since his shine spike is mcuh riskier now, and fox made much more use in melee of straight up running shine approaches than falco, which used to be able to clank out anything. additionally, their are wayyy more characters now with good projectiles and ways of dealing with projectiles and even healing from energy based ones. falco can usually out do most characters with projectiles since his are pretty much always better. fox cant on the other hand since his dont cause hitstun. This can cause a sway in this MU spread.
you could also argue on the other hand that better recoveries in general make it harder for falco to edge guard than for fox, and that better edge guard games in general mean even more absolute death offstage for falco than fox.
in all seriousness though, i wouldnt focus on whos better.. theyre both two of the best characters in the game and can deal with any character efficiently , and if you play them intelligently theyre more than a force to be reckoned with. focus on your own abilities with the characters you like instead of which character is better