I think that's just the way it is in ebooks. It'll take time to fill in all the missing books. Some will never get filled in.
I keep an eye out for certain books I read back in my youth that haven't been available as ebooks and every now and then one will show up. Sometimes whole authors are missing even though they were huge best sellers when I was young.
One of the best selling authors in paperback in the 1950s and 1960s was Frank Yerby. Most of his were precursors to today's romance novels, which hadn't become a thing yet. But he also wrote some really good and well researched historical novels and, except for a couple based on the New Testament, they're simply unavailable.
John O'Hara, one of the most respected and best selling writers in that same period, was also unavailable till a couple of years ago. Then someone decided to publish his books, finally.
It just takes time. It's the nature of the business that they can't do it all at once. In some ways that's kind of fun, although it really can be frustrating at times.
I just bought a paper Frank Yerby book and a paper Frank G. Slaughter book, both favorites of my youth. I haven't read a paper book in well over a decade. Maybe two. I'm not sure I'll talk myself into reading these but I'll try.
One thing I'd forgotten about paper books was that there's no front light. How aggravating.
Barry