Royalties are sometimes complicated by the time factor -- some songs may be Public Domain, or Trad., or it may be unclear who wrote them (the first person to record a song wasn't necessarily the author). Artists can also claim arrangement credits.
But there are many clear thefts.
Some of the original artists did get compensated, but it usually involved lawyers.
Rev. Robert Wilkins, who was still alive when the Stones covered "Prodigal Son" (a/k/a "That's No Way to Get Along") eventually got royalties and a composer credit, as did Mississippi Fred McDowell for "You Got to Move."
Cream paid Skip James for "I'm So Glad," which covered his medical bills and pretty much saved his life.
"Goin' Up the Country," based on Henry Thomas' "Bull Doze Blues," had new lyrics written by Alan Wilson, which should have garnered him a co-writer credit, not sole-writer credit. I'm not sure Thomas left any heirs, or if there was anyone to fight for royalties. (The song may have been PD by the time Canned Heat recorded it anyway.) But in any case, Wilson didn't live long enough to see any royalties himself, being one of the "27 Club." Wilson's family has done very well by it, however.
Led Zeppelin "borrowed" a lot -- they might be the worst offenders -- Robert Johnson, Memphis Minnie, Muddy Waters, Sonny Boy Williamson, Howlin' Wolf, Willie Dixon and many others had things appropriated by Zep.
Worst of all, they (or their managers or label) only paid out and gave proper credit kicking and screaming. Lawyers eventually caught up with them; artists were paid, and current versions of the albums credit the appropriate writers.
Except one: Page's "Black Mountain Side" is Bert Jansch's "Down By Black Waterside." Jansch only died a few years ago, so he should have enjoyed royalties on that song for some four+ decades, which would have amounted to a pretty penny indeed. But though Jansch was fully aware of the theft and was urged by friends and record labels to go after Zep, he couldn't be bothered.
Jeez Louise, if you love a song enough to record it; it you expect to be credited and paid for songs you composed -- do the right thing!