An explanation of why this type of static cache pattern is an anti-pattern or bad?

Imagine you need a lookup cache for something.

I know this implementation is wrong. For one thing, you cannot mock it for unit tests.

But I know there must be other reasons it is bad.

public class FooCache {

  private static Map<String, String> cache = new HashMap<>();

  public static populate(...) {
     ...
  }

  public static String getWhositForWhatsit(String whatsit) {
     return cache.get(whatsit);
  }