The Senate Guard any better?

Most of them were just Padawans or young Knights who were recruited either because of them demonstrating a disillusionment with the Jedi or were coerced thru torture, not because of how formidable they were. They often face opponents of equal or greater skill, and they were considered expendable by the Vader and Emperor; useful tools for rooting out a stray Jedi nuisance here and there, but leaving the big dogs for Vader to handle.

Reminds me of the purpose that Ashigarusha being the expendable soldiers to deplete all of the enemies ammunition and weaponry until the real soldiers arrived.

Palpatine does not want them to become a threat.

And they are still pretty dangerous as long as they don't face enemies like Ahsoka.

Not all of the Inquisitors were weak. The Grand Inquisitor wasn’t weak by any means.

Asides from them being expendable to Palpatine and Vader, they were still a serious threat to most opponents. We see them obliterated by characters like Ahsoka and Maul as they were not powerful enough to counter opponents of that level.

However, they were still formidable threats to most other Jedi such as Ezra and Kanan. The Grand Inquisitor was a superior combatant to Kanan, but a fearless Kanan instilled so much fear on the Grand Inquisitor that he was overpowered and defeated. But before then, Ezra and Kanan constantly struggled to survive duels against the Grand Inquisitor. They later found the same issues fighting the Fifth Brother and Seventh Sister on multiple occasions.