I still generally have a hard time believing the literal best of the best struggles to get results. Basing it all on something as nebulous as "potential" seems rather dubious. Especially when its yet to consistently be pulled off. It also seems to kinda fall into the pitfall of regurgitating popular ideas without much merit so that they have become accepted. Theory crafting is well and good, but repeatedly claiming Pikachu is better than the several fighters with better and more consistent results...why? At least Joker proved it pretty consistently even if those results are largely aging. Pikachu has a few good showings from ESAM, but also some not as good showings from the same. Nothing really screams overall best in the game.
In the end, we are not dealing with perfect situations and perfect players, so even if the Pokemon was best in theory (and let me stress "if") we have little to nothing to actually indicate that is the case in the real, practical world.
And generally speaking, the roster is so huge, and odds are their personal experience is still relatively limited, so trusting even a handful of pros to make an accurate tier list doesn't seem like the best strategy. Even if they are legitimately trying to make an accurate one, they probably just don't know enough. Especially about lower tiers, but ironically enough, how often do most fight a Pikachu? Even more, how often do they fight one that isn't ESAM? As tired as it is, remember, if Leo winning doesn't make the Byleths top tier, then ESAM (being less consistent) doesn't make the rodent the literal best in the game.