Why do Java programmers like to name a variable "clazz"?

It's simply because 'class' is a reserved keyword, hence Class class isn't allowed. Therefore you'll see Class clazz or Class cls.

Because they cannot use the word they want to use which is class. It is reserved.

clazz has been used in Java in place of the reserved word "class" since JDK 1.0. "class" is what you want, but abbreviating or inserting junk ("a", "the", "_", etc) reduces clarity. clazz just says class. "International" English speakers (those reading both British and American English) are used to transposing 's' and 'z'.

Since Java has had disclosed source and a suitable culture right from the start, worthwhile Java code and tutorials pick up the same conventions. That's one of the great things about the Java ecosystem, which I think has been an important part of its success.