Why do fighter aircraft manufacturers still focus on making their planes more dogfight-worthy?
That same thinking, "dogfighting is dead", got the USAF and USN in serious trouble in Vietnam. The U.S. Armed Forces were convinced that the next major war would be against the Soviet Union directly, in theaters including Europe, Alaska and Canada, and as a result, fighter designs succeeding the very successful Sabre and Super Sabre day fighters used in Korea began emphasizing standoff capabilities, believing the aerial threat would come from various classes of nuclear bomber, calling for fast, long-range missile-armed interceptors as a counter.

That war, thankfully, never materialized. Instead, the U.S. was pulled into more or less another Korea, a proxy war fought between Soviet-supplied and trained Communist forces against a U.S.-backed fledgling democracy. The big, heavy U.S. F-4 Phantom interceptor and F-105 Thunderchief light bomber, neither of which had internal guns, found themselves facing the older like-minded successor to the MiG-15s they faced in Korea, the MiG-17, coupled with the MiG-21 close-range interceptor. Instead of the long-range standoff air war the USAF was built for, US pilots were ambushed at close range by guns and infrared missiles, resulting in a piss-poor 2:1 kill/death ratio in aerial engagements, compared to the 14:1 ratio they enjoyed in Korea.

The result was increased attention paid to "air combat maneuvering", and an emphasis on this style of combat in the next generation of fighters adopted by the U.S. military. The F-14 and F-15, though intended to bring back dogfighting (and they did), turned out to be big, fast interceptor-style fighters like their predecessors, that still only had a clear advantage in BVR engagements. So a group of USAF generals, seeing what its allies had been doing with the F-5 Tiger that the Joint Chiefs had more or less passed on (a few trainers, a few aggressor planes, but no combat adoption), commissioned the LWF competition, which would produce the F-16 and F-18 lightweight multirole fighters with an emphasis on the close-quarters maneuverability that the Communist powers never really forgot. 

The F-22 splits the difference between the F-15 and F-16's air combat strategies, producing a heavier but highly maneuverable fighter that excels at air combat at any range (but is so expensive that so far the USAF have only deployed them in situations where they would never face any real threat). The F-35, however, is rapidly becoming a disappointment; compared even to the F-16 or F-15E it will replace, it "can't climb, can't turn, and can't run", and the A variant is the only one with an internal cannon (which carries only 182 rounds of ammunition compared to the F-22s 480, the F-15's 650 and the F-16's 510); the STOVL B variant and the Navy's C variant won't have guns at all.

In today's military theater, the U.S. enjoys "air supremacy"; we're fighting largely paramilitary forces with practically zero air to air capability. Tomorrow's theater, however, may be quite different. Diplomatic relationships with a number of Russian-supplied nations including Russia itself are souring day by day, and the U.S. could easily find itself drawn into conflicts over Taiwan, the Korean peninsula, India/Pakistan, the Ukraine, etc, where they'd face militaries fielding Russian jets purpose-built to mix it up at visual range.

In short, while the U.S. hasn't directly fought a conflict against a serious air threat since Vietnam, there are a lot of potential near-future conflicts in which they would get their chance, facing the Sukhoi PAK-FA or Chengdu J-20 with skilled Chinese or Russian pilots at the stick. Even if these fighters aren't fielded in mass numbers, the U.S. would still face the Russians' direct answers to U.S 4th and 4.5-generation fighters, including the MiG-29 and Su-27/30/33.  Many variants of these were specifically designed to out-maneuver the U.S. F-15, F-16 and F-18 (which would likely be the first jets in-theater), and which would still be a match for the F-22 in a visual-range "furball".