IMO, its all about money. These programs are written for the masses not for the relatively few high level modern composers. A huge part of their market is music schools/teachers/students as well as, just as JBM mentioned, Pop, Jazz, Film, Theater, Recording Studios, etc. Every business tends to cater to the needs of the largest part of its customer base and, for music notation, that means standard practices of the general music creating public.
I'm not saying this is bad or good. However, its quite logical and to be expected. For a company to spend the time and money required to fulfill the needs of a group they would think of as "specialists" would only make sense if there were enough of them to make it worth the company's while. Even though you may only speak with people who do the same kinds of things you do, that doesn't mean such a group is large in the grand scheme of things. From a business point of view, its all about filling the needs of the many rather than the few. Otherwise, they won't be in business very long.
In time, I think these programs will reach the point where anything we can imagine will be possible and even easy. It's just that, for the more esoteric features we're probably going to have to wait a while.
Be well,
Jimmy