Ash has had only two constants throughout his journey in Pokémon: Pikachu and Team Rocket. At this point, "twerp" comes off as an affectionate nickname, as Jessie, James and Meowth follow Ash on every step of his adventures. The world of Pokémon has become so vast, it is beyond even cartoon logic that Team Rocket would still be after Pikachu, as there are hundreds of Pokémon more valuable than one overpowered mouse that isn't even fully evolved. In fact, they feel like strange friends at times.
Ash has known Team Rocket for so long that it feels like there is some sort of unspoken bond between them. After all, when they are in trouble, Team Rocket doesn't hesitate to ask for Ash's help, nor does he hesitate to give it. They are as likely to team up as they are to be adversaries, to the point that they don't even feel like enemies anymore. Interestingly, Ash never even feels the need to warn any of his new travel companions about Team Rocket.
Whenever anyone asks who Team Rocket is, Ash refers to them as bad guys who steal other people's Pokémon; although, they have never successfully stolen anything. Even Ash's trademark exclamation "Team Rocket!" isn't believable anymore. He doesn't sound surprised or concerned; he almost sounds excited. Ash being who he is, Team Rocket could probably drop the charade altogether and just join him and his friends.
Team Rocket's most often used approach to stealing Pikachu is to appear in the middle of nowhere wearing disguises. Sometimes their Team Rocket uniforms are visible beneath their costumes as they claim to be offering the exact service that Ash and his friends were just privately discussing. Ash may not be the brightest Voltorb in the Pokédex, but even he can't still fall for this routine after almost 25 years.
The most probable explanation is that he is, in fact, not falling for it. Ash likely knows that Team Rocket is always lurking, and they don't change up their tactics very often. Considering they've never actually made off with one of his Pokémon or even caused any lasting damage to others, they are relatively harmless, and Ash seems okay with letting them be who they are while getting some training in the process.
Team Rocket's meddling might just be their way of helping Ash, providing obstacles for him and his Pokémon. They either have a subconscious desire to fail, or they aren't actually trying to steal Pikachu. Whenever they successfully snatch Pikachu and escape, they leave their prize unattended so they can celebrate, rather than hightail it back to Kanto and Giovanni. Even Team Rocket can only fail so many times before it feels intentional.
However, in Giovanni's presence, Team Rocket takes their job seriously. Fear of their boss is likely what keeps them from dropping their faux antagonism because when they're free of his oversight, they're themselves. In fact, Jessie's penchant for entering various contests with undying support from James and Meowth is indicative of their own desire to have adventures just like Ash does, and it's something the boss likely disapproves of.
It's far more likely they're fixated on Ash because he is living the life they wish they could. Thanks to their jobs, the closest they can come to their own journey is following Ash on his. They are as invested in his success as the audience at this point, as though they are living vicariously through him. They even have a habit of joining the arena staff for League Championships so they can watch Ash's matches.
As for Ash, he appears to feel sympathy for Team Rocket, knowing they're never going to change. The least he can do for them is play along with whatever helps them feel better. If he doesn't outright despise them at this point, he's never going to. He refuses to see anything but the best in people and Pokèmon, and deep down he knows Team Rocket is capable of good.
Ash and Team Rocket want the best for one other, and they also accept each other. Their relationship has always been at the core of the series, and it doesn’t appear to be a dynamic that's going to change anytime soon. Even Ash's friends follow his lead and accept the criminals as allies when need be. Pikachu might be a tad less sympathetic as the victim of this strange friendship, and the Pokémon doesn't appear to hold back when using Thunderbolt to launch Team Rocket into the stratosphere, but Pikachu doesn't speak for Ash.
