Perhaps because you won't use them against the only theoretical enemy around your parts, but against a very real enemy in other parts of the world as part of a NATO/UN operation. If I recall correctly you guys sent about half a squadron's worth of F-16s to Italy to support NATO against Libya. Against Libyan air defenses the F-35 would be far safer to operate than any 4th gen fighter-bomber. Likely you will send aircraft to operate with NATO against many other 3rd world nations during the next 50 years, but only in small numbers. If, as you say, you won't stand a chance against the Russians no matter what aircraft you buy, then why not buy one that that works best in the military operations you actually do commit to?