I think Nintendo does realize that. They've only gotten the smallest of buffs, nothing that really had a significant impact. More than just brushes though, there's an entire category of weapons that are popular in casual play but have remained bottom tier for nearly their entire lifespan: Aerospray, Sploosh-o-Matic, Clash Blaster, Undercover Brella, Bloblobber*, etc. I do think Nintendo deliberately plays favorites with what weapons are allowed to be good.
(Yes, Blob was good on release, but after they nerfed it into the ground it was never allowed to return)
Main weapons do not exist in a vacuum, you have to look at the kit as a whole. The design problem with Aerospray is that it can't do much else besides farm special, and its value is entirely dependent on what special it's farming. It's so bad at doing anything else that it's actively disincentivized from trying to engage with the enemy team, just farm because that's what you're good at. You can't fix that without completely redesigning the weapon into something that can fight, at which point it isn't Aerospray anymore. We already have Shooters that fight.
The problem with this is that it's a playstyle that sounds appealing to beginners, because it lets you feel like you're contributing while avoiding having to interact much at all with those scary opponents. But it's also one that would be very unhealthy for the game if it was allowed to be good, because it's not good to actively avoid interaction like that. See also: S2 Armor farming.
Ideally, Nintendo would never have put this type of weapon into the game in the first place. It'd be best to have a game where every weapon deserves to be competitively viable. But since this is in the game and Nintendo doesn't want to cut main weapons ever, they've opted to cover up the design mistakes by just leaving them in the trash. Let the casuals have fun with it in Turf while the top level meta remains unaffected.
The fact that it's consistently been at the bottom since S1, I do think Nintendo left it there on purpose. I think playing favorites with balancing is something game devs do more often than people realize, and I think it can be justified.