I dunno… I don’t think I was “disinterested” when I was teaching, nor were any of my fellow grads, at least not most of them. But I guess you’d have to ask my students about that…
At my school anyway, part of the problem is overworked lecturers, who are teaching the majority of survey classes (tenured faculty is expected to teach survey classes, at least one a semester). Most of the upper division stuff is taught by tenure/tenure track faculty or senior lecturers (we have a lecturer track in the history department here).
Much more of the undergrad, survey type classes in recent years in many smaller colleges/universities without grad programs are taught by adjuncts, with little to no job security, as opposed to grad students. Larger universities depend on grad students, and depending on the program they are overworked and some of us are underpaid (my department gets nothing near a living wage).
And @anon67050589 is correct about many tenured faculty not giving a shit about undergrad surveys (and in some cases, about upper division and even graduate courses). Then again, I’ve known lots of passionate profs who care about their students and work hard to make sure they get a good education.