Again, there's a much bigger war going on out there so they'll have no shortage of volunteers to be soldiers or even pilots. The ones that survive more then five minutes may even get promotions to squad leaders, Captains or even Commanders. People with long experience are more valuable and are needed more and more to manage things at the strategic level as more worlds join them, as territory gets occupied, worlds liberated and mass migrations of refugees.
Sure, you can handwave it after the fact, but it still rings false that the few characters who happen to be the series leads also happen to be the ones who are best qualified for those leadership positions, especially when they come from such disparate backgrounds and haven't really done all that much for the Rebellion. Leia, of course, was already a leader, and Luke would've made his name with the Death Star and been a committed resistance fighter from then on. But Han was at best a mercenary for the three years between ANH and TESB. Leia's line "I thought you decided to stay" suggests that it was a decision he'd made fairly recently, suggesting his affiliation with the Rebellion was rather tenuous and that maybe he'd come and gone in the intervening period. And then he spent six months as a wall decoration, which is not the sort of career experience that generally leads to a promotion to flag rank immediately afterward. It's just not very credible, and it smacks of sexism that he had to be equal in rank to Leia despite deserving it far less. Why couldn't he have been a colonel, even?
Lando isn't just some smuggler, he's the former Baron of Cloud City. Someone used to leadership, managing logistics on a large scale and with the wherewithal to do so for years without drawing attention from the Empire or the Mining Guild. That's valuable experience in and of itself and whatever he did at Taanab clearly convinced Alliance High Command he also has what it takes to lead people into combat.
Civilian leadership experience doesn't automatically translate to military experience, any more than being a businessman qualifies someone to be a president or cabinet member. It's apples and oranges.
That's when I remind myself it is make believe.............
Exactly. It's overthinking it to try to rationalize it as some kind of defensible, logical military practice. It's an arbitrary choice made in an '80s adventure movie aimed at children. It was done purely because these were the lead characters in the movie and that mattered more than any kind of internal sense. It's no more logical than a bunch of stone-age teddy bears being able to defeat the military elite of a galaxy-spanning technological empire because they have good and right on their side.