Because it costs money to do so. It’s designed to deter employers from just hiring all foreign employees at a wage and encourage employers to hire domestic employees. If employers had their way they’d hire only international applicants since they will more likely be wiling to bend over backwards. If you are on a work visa and are fired, you are very likely to be forced to leave the country; employers know this so they take advantage of that. The government discourages this by disincentivizing employers from hiring foreign workers on a visa by making the process expensive.
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