Maybe the reason is just insecurity. Insecurity of seeing "Party games" as lesser, causal trash or maybe something prove to the Big Boys Club at FGC that Smash hang with rest of them. I don't know
Based on the way you've talked about this before and the way you phrase this now, this really seems like a personal problem. It's more like you personally don't want Smash to be classified as a fighting game, you seem very spiteful toward the idea. The way you're accusing people of being adverse to admitting Smash is a party game, just in reverse. It doesn't seem like this is a good faith argument and it's a bit silly to hear you try to project it onto others.
I'd like to say also that nobody is denying the party elements of Smash Bros. There's just no reason why a "party fighting game" can't also be a thing, or isn't already very much a thing. You play as a character who goes head to head with other characters, you punch and kick and aim to knock the opponent out. It also has wacky items and random events to spice up and interrupt a match. These aren't elements that anyone is denying exists, but rather most people here agree that this doesn't not disqualify Smash from being a fighting game. If you want to discuss whether or not Smash Bros is a "serious" fighting game, one that should be held in esteem in a competitive or tournament setting, that's another conversation. But those aren't the only games to qualify for a wide encompassing genre which, like many other popular genres, evolves and spreads out into different avenues. Just as there are action RPGs, or puzzle platformers, and so forth. Smash, maybe by accident, pretty much invented a new subgenre.
Is ARMS a fighting game? What about Power Stone? WWE? None of these play like Street Fighter, but I think you'd have a hard time denying they are classified as fighting games. No amount of quirkiness or simplified mechanics really take that away from them. They won't be center stage at EVO, but that doesn't really matter. The intent is clear.
"Party game" is more of a description than a genre - there are games of many genres that appeal to that high energy group environment. Is Mario Kart a racing game? Of course it is, nobody in their right mind would say it isn't, but it's not Gran Turismo. Is Smash a fighting game? Debatable apparently, but it is also a "party game". Nobody should be denying it, I haven't seen anyone here do so either. That's just not a category Smash feels any need to lock itself into, measures have been taken to balance it further and appeal to players who treat it more like a competitive fighter. Sakurai has outright talked about frame data and made a self aware joke about it. Like it or not, the series has willingly adopted this expectation and has played further into it.
So with all that said I feel like the series will continue to speak for itself. Smash's ability to be whatever game people want it to be is probably its greatest strength and whatever genre it is, the only thing that's really important to me is that I love playing it.