A lot of Captain Marvel and the Marvel Families early adventures slipped into the public domain before DC bought them (Public Domain laws were different at the time), but the rest of them and everything Fawcett DC Does currently own. I think they originally leased the character when they published his stories in the 70’s but they have since bought them outright.
The short answer to why they are not on DCU is that they are not in the digital library. I have no idea why his Golden Age stories, and for that matter his series from the 70’s are not digitized though. Since a lot (I believe most) of his Golden Age stories are in the public domain they have not been hard to find, and due largely to the misconception all Golden Age Fawcett Comics were in the public domain some public domain archives had all of them. So it is debatable by now most who wanted to read them haven’t found them, the number who would pay for Golden Age comics just for the story (I.E. what you get if you buy them digitally) is not a huge number compared to who would buy digital comics of current stuff.
But I do hope they archive some, it just seems a shame not to have the Golden Age books available in the DC Digital Library at all. And am genuinely surprised why nothing from his 70’s run is there.