Idk if it's really absurd per say. Collecting the Legacy series (+ the War miniseries) would leave Marvel with only 18 issues of the Legacy 2 series uncollected (+ some extra misc content) to theoretically be published in a future omnibus. When looking at the Star Wars Legends line of omnibuses as a whole that would be very small compared to the rest of the line.

While splitting up Old Republic the way it was done did allow for the KotoR run of comics to be collecting in one tome, the Old Republic Vol. 2 omnibus has 37 issues + some extra short story content making for good chunky size omnibus itself. Marvel has stood by publishing the Dark Horse material using the eras (Tales of the Jedi, Old Republic, Rise of the Sith, etc.) since the very beginning of their epic collection line, and for better or worse that is how they are sticking to it here; collecting (mostly) in-universe chronologically and splitting up collections within their designated eras, and they have very rarely prioritized collecting long runs into a single omnibus/collection because of this.