Pikachu relies on those "over-extensions" to actually get anywhere because he needs to make some kind of read to get a kill. You have to take risks as this character. You can't play it totally safe. ESAM may make mistakes, true, but you should consider the opportunity cost of not making the risky plays that he does. His opponents will live to enormous Rage-enabled %s and kill him with a simple poke.
Pika being high tier is incredibly suspect. What matchups does he win against any relevant character? He seems to beat Falcon pretty decisively, yeah, sure, and maybe you can tack on DK too, but here are characters who in the last year have some claim to either even or better matchups against Pika:
Here are characters who have been theorized to lose to Pikachu but haven't, in practice:
Some more obscure matchups like Pit and WFT are probably slightly in Pika's favor, but who's left beyond that? Duck Hunt, Falco, Samus? These characters lose to a lot of other characters. Pikachu definitely does not have the matchup spread of a high tier character.