TBH, flat-measuring speakers can be easy to achieve, but it's going to rely on selecting well-behaved drivers in a properly designed cabinet and ensuring that your measurements are done correctly, (and time aligned, if using a crossover design software like XSim) and that your drivers can actually meet one another at the desired crossover point.
We've seen manufacturers who selected woofers which had breakup issues at or before the crossover point, and that's not really fixable.
We've also been sent custom speakers where the woofer naturally rolls off before it can ever reach the chosen tweeter.
It's why things like canned/calculated crossovers tend to be such a mess 99% of the time.. There are several factors that are often not being taken into consideration..
Baffle-step, edge diffraction, acoustic spacing, cone breakup, just to name a few.
A lot of times for manufacturers, the main things to consider for the average person are price and looks, everything else is often secondary, so long as there are no glaring flaws. (Though that's not always true)
If a speaker is going to retail for $1000, they have maybe $200-250 in the drivers, crossovers, cabinet and packaging.
It's all a balance of tradeoffs, even with full-range drivers.
In the end, especially within the retail space, it's largely about what appeals to the most end users.