Ummm because the highest form of canon SHOWS Mace DISARMING Sids in Sabers. It then also shows the only person being overpowered in the froce is again SIDS.
Indeed, "highest form of canon" sounds cool but with this statement you can go to film only threads, here we discuss EU and consider all feats.
In TFU book Marek grabbed Sidious by wrists and made him taste his own lightning like Mace. Not only this struggle lasted relatively as long as with Windu but it culminated with enormous explosion.
"No!" the apprentice cried, dropping his defenses to strike one last time at the Imperials. Energy surged through him. He felt as though a star had blazed to life in his chest. Driven by concern for his friends rather than himself, he embraced the Force completely, utterly, and was rewarded with strength that made his efforts with the dark side look like those of a child. His nerves were on fire. Streamers of light radiated from his skin. His bones glowed like radiant lava.
He saw rather than felt the massive shock wave that consumed a large portion of what remained of the one observation dome. A glowing bubble of fire tore the stormtroopers to shreds and engulfed Vader and the Emperor. Shrapnel filled the air like dust caught in the beam of the Death Star's powerful laser.
...
Weakened by the blast, the dome's supports gave way. It collapsed into the superlaser dish, triggering a series of conventional explosions. Stormtroopers converged on the site. Through the dense smoke, two figures were visible from the apprentice's rarefied perspective.
Darth Vader struggled to his feet from the rubble, even more damaged than before. He reached out for support and found only his Master, scowling.
The explosion torn troopers to shread, caused a dome to collaps and even damaged Vader. Sidious was right next to explosion, yet, he didn't even lose his footing like Vader and was seemingly unaffected.
If Sidious can resist such a powerful explosion after suffering from his own lightning, then he definetely can resist lightning for much longer.
You could still argue that he became more powerful since RotS, which imho is true. Yet, his body at that point was weaker as he was much older and the darkside was decaying his body from within.