I think that it was established fairly early on that Pikachu isn't necessarily supposed to be viewed as especially powerful in the anime, recall how even well into the Gym thing Ash wasn't able to win Gym badges legitimately, despite the promotion around Pikachu being then at its highest. There was also a strange case where they couldn't beat female Gym leaders, which seems weird as it was clearly not an attempt to cater to the primary audience of the anime at that time, but perhaps they were just infatuated with females. Anyway, since then the anime has generally been less popular, and hence Pikachu's strength might be a bit random at times (in the movies it's generally a bit senseless, and really after a Pikachu straightforwardly makes a Golem faint with an electric-type attack Mewtwo could really have done whatever with most of the world), but it's probably telling that even at the height of its popularity and so on it was generally played down, especially relative to other apparent concerns of the people writing the anime, and as such if it shows 'strength' this is probably more credited by the anime to Ash than the Pokémon itself, which is consistently marginalised in newer teams.
I think there might also be a slight degree of reluctance to acknowledge that a symbol of the series is a Pokemon from R/B and unevolved, since when they haven't really managed to come up with any lasting symbols of it. Other than that, looking back at for instance Gary's fight using an Electivire, it does appear that despite the time it's been used, Pikachu is still easily outdone by electric types and so on which face it, and you'd dread to think how much Pokemon could cause it issues if it had a type advantage, which seemingly rarely counts when a Pikachu has it. Honestly it's usually only kept afloat because of Team Rocket, who aren't actually assumed to be that strong mostly, other than which it has a fairly poor rep. It hasn't really had that much of an independent plot-line since the first series of the anime, and as such Pikachu isn't necessarily assumed to be that much stronger than the Pokémon of that stage. In which sense its main 'development' there was mostly just that it stopped using Thundershock on Ash, well, and moreso on Serena, and still used it on other Pokemon somehow without a crippling sense of shame, but in any case the Pikachu itself doesn't generally get to do that much which is particularly assertive, although it's strange that, outside of Team Rocket, it was mostly of note for being tamed and then not defeating gyms. Perhaps it should go solo, and throw caution as it were to the wind. After years of being weak to every type electric-types are strong against, except on occasion to flying-types, it could do with a break to pursue its true rival which is not Gary but the electric type itself.
