Because primitive recursion is quite easy, and so it is quite easy to get a universal Turing machine. Filling that machine with a useful program is another thing entirely, but that's why we have evolution and programmers...
Something that also makes this point is AIXI. All the complexity of human-level AGI or beyond can be accomplished in a few short lines of code... if you had the luxury of running with infinite compute resources and allow some handwavery around defining utility functions. The real challenge isn't solving the problem in principle, but defining the problem in the first place and then reducing the solution to practice / conforming to the constraints of the real world.
If it's worth saying, but not worth its own post, then it goes here.
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