>>111937685To be fair given the omnitrix is literally tech created by the greatest genius of a race of hyper intelligent aliens and They had intended to capture it before it bonded to a host it's believable they wouldn't have much in the way for containment outside of what they could build isolated from their vast network of supply.
What you should be asking is why they didn't just lop his arm off and surgically remove the Omnitrix.
Though that's also rhetorical because both questions can be answered with, "Because it was a cartoon aimed to sell toy to 10-12 year old boys in the mid 2000's"