A governor has to take care of his own state first, and vie for national attention on time left over. A Senator, on the other hand, addresses the nation every time he or she steps up to speak on the Senate floor.
It is the nature of the job of US Senator to take on a sitting president of the opposing party, especially one so destructive as Trump has been from day one in 2017, let alone during his second term. Men like Jon Ossoff and Mark Kelly have a built-in access to national exposure if they want it. A governor has to build his or her own national stage, especially if they are from small states. You have to have both an ability to inspire and a lot of drive to become a Howard Dean or a Bill Clinton.
In a way, the many good Democrats running in tough re-election races are fortunate that the race for the White House isnt until two years from now. If they come through this midterm test of fire, they will be all battle-tested and will have nothing to lose in 2028. If Rob Sand wins as Governor of Iowa, he will not be seeking the White House in two years, but he will definitely still be young enough to try for it in ten years, or even six if some catastrophe should occur in 2028.
Speaking of 2028, I think I already pity the Republican candidate who will be debating Kelly, Newsom, Ossoff or Shapiro, if our nominee is one of those four. I think any Republican would have a better chance of pushing over the Washington Monument with one hand than they would of scoring a debate win over any of those guys.