One of the Big Lies that they repeated over and over throughout the election season was the one about the "border crisis," where allegedly criminals and rapists were flooding into the country across the southern border by the millions. They promised to round up and deport all of them, literally millions of people. You can see where the problem arose: Lies collided head-on with reality. There simply are not millions of migrants for ICE to round up. They don't exist. They never existed. It was all a lie.

Now, the regime has to appear like it's Doing Something(tm) by actually deporting people. Stephen Miller has even given ICE a quota of 3,000 deportations a day, and it's struggling. They have to make the numbers somehow, and the low-hanging fruit are the immigrants that they already have records on, and know about. ICE can just comb the immigration records, and go pick up people whom they know exactly where to find. Morality and logic have nothing to do with it, it's all about throwing the red meat of performative cruelty to their base, and intimidation to their political enemies.

Why would missing one appointment even be grounds for deportation for anyone in the first place?

Because evil.

Because the goal is to remove brown people, and immigration status is an easy way to start doing that. They will move on to a more Permanent Solution as time goes on.