What textbooks are you referring to? I took multiple history classes in college that covered Korea, I just think you need to broaden your horizons.

East Asian Studies is a starter.

A lot of this has to do with the fact that in the past western watchers came to the subject of korea after starting on chinese or japanese history. Whether acknowledged or not this biases the reader when they then turn to korea and many regarded korea as being of little interest because it was regarded incorrectly as either a subject or vassal of china or a nation that was always a "shrimp between two whales".

Those two summations are often cited often in past works, with decreasing frequency of course as the numbers of korea specialists increase the closer we come to the present time, but broadly speaking the former is due to the assumption in chinese history that it was the centre of the world and everybody was it's subject and the latter was due to the misapplication of a korean proverb and the assumption that japan was regarded as a "whale" throughout all its history (correct post-1895 but certainly incorrect prior to that).

In either case it hardly encourages a scholar to spend time getting to properly understand korean history, and naturally leads to less material being available in english or another western language.

On top of that much of that material that took precedence in the mind of westeners during the period leading up to 1905-1910, when japan took effective control of korea, and leading up to independence was plain propaganda because it was written by people who had no actual first hand knowledge of korea. Take the example of Andre Schmid's article "Two americans in Seoul" comparing G.T.Ladd's "In korea with marquis Ito" vs Homer Hulbert's "The passing of Korea":

This article focuses on two prominent American writers who, during the Protectorate Era (1905-1910), came to very different positions on the question of Korea’s future. One of these authors, George Trumbull Ladd, is renowned, even notorious, for his book "In Korea with Marquis Ito", as well as for several articles and letters in the New York Times, in which he offers arguably the least nuanced support of any of his contemporaries for Japanese actions on the peninsula. The ambivalence that underlay the views of many of his compatriots writing about the rise of the Japanese empire found no place in Ladd’s unrestrained and enthusiastic approval. At the other end of the political spectrum lay Homer Hulbert, a long-time resident of Seoul, who, in his major work of the era, "The Passing of Korea", explicitly presented himself as an expert with extensive experience inside the country, in order to justify his argument against Japanese rule and his criticism of the representations of Korea offered by many contemporary travellers.

Ladd's work was particularly pro-japanese but one can take it as indicative of the scholarship of the time. In contrast, the work of people with direct experience in korea like Hulbert and F.A.McKenzie were disregarded. Whichever way you look at it it's not particularly inducing towards people being interested in korean history. Today the view of the two authors is reversed.

Material on korean history is there but typically you'd need to look for works specialising in korean history.