Only Truman served in combat and Eisenhower was stateside during ww1. It just seems to have jumped generations very quickly. In comparison Kennedy to HW served, although Johnson and Reagans service was not combat. Is there any historical reason for this? Roosevelt taking up 13 years for example? Different sentiment towards military service in the interwar period?
We are also likely to not have any vietnam combat veterans serve as president if anyone wants to touch on that
The scale of the US's involvement in WW1 was considerably smaller than in WW2. In WW1, the US mobilised about 4 million personnel (of which only about 2 million were actually sent to France and fewer made it to the front line) and was involved in the war for only 18 months. In WW2, the US mobilised about 16 million personnel and was at war for three and a half years. So a large part of the answer is that the fraction of American society who had seen action in WW1 was just a lot smaller than in WW2.
The second part of the answer is Franklin D Roosevelt, and his extremely long tenure in office. Many of his political opponents were from the generation that had been drafted (Huey Long dodged the draft, Alf Landon was a captain in the Chemical Corps but wasn't sent to France in time, Wendell Wilkie was deployed to France in September 1918, too late to see action). But Roosevelt was the man who won.
Then you have Truman and Eisenhower, who did indeed serve (Eisenhower's experience is arguably representative of the average American WW1 soldier in many ways, since the vast majority didn't see action). After Eisenhower left office in 1961, US politics favoured younger men such as Kennedy and Nixon, which essentially shut out the ageing generation who might have been drafted in WW1.