Part of it is the appeal of being the badass that can beat up/stomp anything, which makes Wario games feel like you're playing as a boss or main antagonist rather than some wimpy hero that's outmatched by his enemies. Mario, Luigi, and many other video game protagonists are the underdog in their story, being weaker and more fragile than many of the foes they go up against. Wario is the complete opposite. He's pretty much objectively more powerful/durable than anyone else in the world, and he knows it.
As for how he's kept likable... well in two ways:
1. He's funny. He gets set on fire, stung, frozen solid, etc for his powerup equivalents, attacks like a goofball towards the other characters and gets thrown into enough ridiculous situations that you end up wanting to laugh with him, not find him annoying or obnoxious.
2. The villains in the series are always worse people than Wario himself. This isn't exactly uncommon with stories featuring villain protagonists or anti heroes (there's a reason Bowser and Bowser Jr's quests in the Mario & Luigi series have them going against other villains, and why the likes of the Punisher go after horrendous scumbags rather than random shoplifters or street thugs), but it keeps people rooting for Wario none the less, since the villains are always much less sympathetic/likable by comparison. Few people are gonna be rooting for Rudy, the Golden Diva, the Black Jewel, Terrormisu or the Shake King.