It's a dishonest comparison.
ARMS, currently, has 7 modes that are playable in a single player fashion, all of which offer unique twists/gimmicks that adapt the core mechanics and one which is local-exclusive, and at least features some kind of semblance of a progression system in the form of unlocking customization options. For what it's worth, it has a proper single player mode at launch, even if the focus is purely multi player. Releasing it at $60 might be a bit of a hard sell for some when several fighting games as of recent like Injustice try to put a bigger emphasis on a "cinematic" element which up 'til fairly recently wasn't exactly common place in the genre. Having said that the idea of releasing a mostly multiplayer tailored experience at a high price isn't exactly unheard of, nor is it something that I think ought to be necessarily condemned in principle. It depends on the way you balance the amount of content that can make up for it, and in this regard yeah - ARMS could use some more characters, some more levels and some more customization options to make it feel bigger. But as a debut title of a new concept, a new IP, I think it has a respectable level of content that would probably be perfect if only it was maybe $10 cheaper.
Street Fighter V on the other hand isn't even in the same realm of comparison. The fact that people are under the impression that it can even be compared to ARMS has me thinking people are sorely misremembering how unfathomably barebones Street Fighter V was at launch. Aside from Ranked/Player lobbies the only single player modes it had to offer was a character specific arcade mode that had cheap production values and could be beaten in 5 minutes, as well as an obligatory trial mode to attempt combos (late edit: survival mode was also a thing but still barebones). The game didn't even have a fucking vs CPU option at release. This was an established franchise that cut it's roster count significantly and compromised on it's supplementary material, and maybe it's lack of single player content wouldn't have been so bad if it at least offered some additional multiplayer modes to cover slack - which it didn't. And yes, it also has "post-launch DLC" like ARMS has promised, but for SFV it's not enough to simply give you free content - it gates the character releases behind in-game money that takes forever to grind in a way to encourage microtransactions. Despite the fact that this was a $60 game it was still way more optimal to pay for getting additional content on top of that, where-as if ARMS is anything like Splatoon, that content is likely just going to be given to you straight up without any nickel-and-diming. If you bought Street Fighter V for $60 at launch, quite frankly, you were ripped off, and I was one of those suckers who bought into it.
I don't think ARMS is a perfect release but there's at least some elements to it that make me think it's merely "could have been better" rather than the disaster shitshow that was SFV. It actually reminds me more of a case of Mario Kart 8 but under Splatoon's content formula. MK8 is a game that is often cited as "worth it in single player" but I never found myself agreeing with this rhetoric - the only reason I buy the idea that it's single player friendly is because the game is inherently a much easier game to get accustomed to and learn. Once you blaze through the Grand Prix cups you don't exactly have much to do on your own except Time Trials. Similarly I think ARMS kind of runs out of things to do in single player in a way but that the core of the game is enjoyable to keep it afloat - it's just that the core is a lot harder to grasp rather than something as easy to understand as Splatoon or MK8, which is going to prevent a lot of people from returning as often as they did here.