In Deadpool & Wolverine, as well as in Loki Season 1, all you got to do to travel illegally across multiple timelines is to snatch a tempad from one of the TVA agents. Like if I were designing such a system, I would have probably put in measures to deactivate a tempad remotely from their admin console or something. It hard to believe someone as intelligent as Kang wouldn't have thought of this.
I hink hubris plays a huge part. If a villain is too smart and methodical, they really can't be beaten, so they have to have flaws. Also makes the characters more human and interesting.
I suppose they have the cop-out for any such obvious improvements of saying that Kang saw timelines where locked down tempads somehow made things worse/didn't lead to his preferred outcomes, so it was intentional to make them so easily abused