Why should you not configure standard JSP tags in the web.xml file?

Because standard JSP tags (for example, <jsp:useBean>, <jsp:include>) are available "out of the box" in the JSP container and do not require explicit configuration in the deployment descriptor web.xml. They are part of the JSP specification. Custom tags, defined by the developer or a third-party library, need to be described in a TLD (Tag Library Descriptor) file, which can then be referenced in web.xml.