For some reason, this appears to have become nearly ubiquitous. Nearly all loudspeakers intended for music listening roll off the high frequencies somewhere above 5Khz. By design. Loudspeakers which are intended for professional uses such as monitoring recordings or mixing sessions are called "monitors". That designation is supposed to indicate (or at least suggest) more extended treble response which was not rolled off by design for more pleasant casual music listening.
I also came across this when setting up my TacT 2.0 RCS Room Correction System. While the Tact does permit you to set up your own custom room curve (a feature I always use) it also comes with a number of pre-configured room curves. These "room curves" represent deviations from flat measured response that are considered to be more listenable than flat response. Nearly all, if not all, of the preconfigured room curves have treble roll off starting well below 20kHz. I have always chosen to extend frequency response flat to 20kHz with a custom curve.
Here is the "optimum" room curve as suggested by Bruel & Kjaer a few decades ago:
Note that it shows a very gradual roll-off that accumulates to 9dB reduction from the midbass to 20kHz.
One rationale given for this is that small rooms reflect high frequencies more than the larger rooms recordings were made in. Attempt to replay music recorded in a larger venue will result in brighter-than-original sound.
A second rationale sometimes given is that recordings are mixed with speakers having typical roll-off. (However, quite likely recordings are mixed with "monitor" speakers not having typical roll-off used in home loudspeakers.)
Anyway, this suggests one explanation for why even though I have set my Elac 4PI II supertweeters at a measurably flat (with midrange) level, the sound may be too bright.
The second explanation is that my overall system response, with or without the supertweeter, is tilted toward the extremes, below 50Hz and above 10Khz. I know of no better way to fix that than with the Tact RCS, which I hope to get to re-calibrating this week.