Really? People fall in love with the Idea of things all the time. It's why games like Bioshock Infinite and Metal Gear Solid 4 take a while to fall in strength despite being way stronger on release. Hype builds expectations, and expectations build an idea of how good a game is supposed to be. It can be hard to let go of that idea even after the game comes out and ends up not living up to your expectations. You've already experienced the game in your mind through hype and discussion, and reality clashes with that.
Smash Bros is an absolute masterclass at building hype. Best in the business. Smash successfully presents itself as this major event in gaming; a Video Game Hall of Fame that brings together all sorts of characters from different games, series, and companies under one product. The trailers are incredible and it's got like 70 stages and 2304890918 songs and woah. Just looking at a list of all the characters in it gives you a feeling that no other series replicates.
But then the game comes out. Do people enjoy it? Of course. They enjoyed Ultimate a lot. But it's still just a fighting game. There may be 70 stages and 80 characters but you probably only play on/as a handful of them. The glitz and glamour of EVERYONE IS HERE wears off. It's hard to feel like you're playing the biggest celebration in gaming when you're lagging online. By releasing, Smash Bros is made mortal. I've spent a lot of time in the Smash community (I do not recommend this), and the very, very strong impression I get from it is that people enjoy talking about Smash more than playing Smash.
Ultimate would still kill most of this contest, and can still easily win its upcoming match, but it absolutely has to bring its A-game against Mario Odyssey, a game which is the consensus Game of the Year if not for Breath of the World. That game isn't just very well-liked; it's adored. I don't think Ultimate is at that level.
I kind of get what you're going for here, but I have to imagine they like talking about Smash so much because they're so passionate about it, which can really only happen by actually playing the darn game.
I mean, MAYBE Melee nostalgia carries it through, but in that case why is Ultimate enjoying the series' best reception since?