Because it adds to the cost. Rounding the edges is highly desirable but I've found that cheaper subs have slots to keep the cost down and most have edges so you get more port noise. Also it isn't just the outside port opening that needs flaring, its also the internal opening.
The author of the above article is correct in saying ports and edges should be flared or rounded to minimize port noise. What he isn't saying is that most slot openings are edged or have a 90deg edged joint inside the sub which is never rounded. Relative to flared cylinder ports, edged slots are a very cheap albeit noisy way to get SPLs.
Ports are a necessary evil in a high powered sub and the best designs avoid edged slots for flared vents in order to keep noise and resonance as low as possible. The Axiom vortex ports are as good as I've seen or heard on any vented alignment.
