It's not like you can just grab a certified plumber from Keyna and they can start here in Canada the next day. There's a ton of location specific information required to be a plumber. Plumbers for example are licensed provincially here, you can't even just move from Ontario to BC and start the next day. You require re-certification which takes a lot of time and costs money. That's assuming their english is good enough to even take the recertification classes and apprenticeship.

Compared to a programmer, who has zero requirement to re-train, it's more difficult.