Measurements are in
Klang & Ton and possibly from some of the builders.
That is not wholly true because we hear a combination of direct and indirect sound. Around a crossover on a flat baffle the larger driver will have a narrow beam width and the smaller a wide beam width. To compensate a non-flat frequency response may be perceived as more tonally neutral. B&W take this to an extreme degree when crossing a large midrange at a high frequency to a non-baffled tweeter as can be seen in the Stereophile measurements despite the 3D directivity plots being nigh on impossible to read values from.
Of course controlling directivity so there is no need to compensate for sharp changes in beam width is preferable if the objective is a neutral presentation. However the "sound effect" of a sharp increase in beam width at higher frequencies can give the impression of more detail and so may be considered a positive attribute by some speaker manufacturers. Maximising a neutral sound clearly isn't the prime objective for most home audio speaker manufacturers. A few audiophiles may seek a neutral sound but I suspect most seek something that sounds good to them. This seems perfectly reasonable given the prevalence of record players, valve amplifiers, etc... which also do not seek to provide a neutral presentation.
A flat frequency response to within a small fraction of a dB is achievable at modest cost if a manufacturer gears up for it (e.g.
preaching tolerances section at bottom). It requires drivers built to tight tolerances, an active crossover, measurement and setting the active crossover parameters for each individual speaker. It also allows the response to be recovered as the drivers drift out of spec with age. Audiophile interest in this sort of "real" technical performance is low. It simply isn't what the hobby is about for most.