Because checks to avoid some kinds of garbage can incure significant performance penalty. There is always a tradeoff in that sense, and I think Clojure is pretty well balanced in that way. So, read the docs, try the functions out to see how they work, write tests, and sanitize the data at the appropriate place.

Matters so rarely that it's far better to rewrite in those cases when necessary.

In core libraries? If low-level functions checked the inputs for each odd case, they'd (often unnecessarily) spend 90% of time doing those checks.