I have to wonder if those slides were used on guns that were used with ammunition other than USGI spec ammo.
No. Most of the ones that I saw came directly from government surplus sales...and that was in the days when GI spec hardball was pretty much all there was. The ammunition was so cheap that nobody reloaded it other than bullseye competitors.
The two that broke in my possession were fired almost exclusively with 200 and 230 grain cast bullets loaded with 6 grains of Unique for 870 and 830 fps velocities respectively.
I have also seen postwar commercial slides that have cracked at the ejection port but every one of them had had alterations to "improve reliability". i.e. lowered and flaired, etc.
None of the ones that I saw had been modified in any way...and the crack almost always appears on the left side of the port in the top corner anyway...where they're not altered. Only two cracked in the right side. One was a slide off a 1918 Black Army Colt that broke adjacent to the first lug wall, and the other one was a cast Thompson Auto Ordnance that broke clean through in the middle of the port wall.
They were just soft. Full heat treating started by Colt in mid 1946, and a good many of those slides probably went to the tail end of government contracts. Those didn't have the hardened recoil insert that was installed in the earlier ones. I've run into a couple of'em.
The contractors probably supplied a dozen slides and barrels for every complete pistol delivered...because they knew that the slides took a pounding, and would be replaced several times over the course of the frame's service life. Everybody worries about the frame, but it's the slide that catches all the hell.
If you have a frame that looks fairly worn, with a slide and barrel assembly that looks good by comparison, it's a good bet that it's not the slide and barrel that came with the pistol originally.