Hi Lionstracs
Studios have been doing this for years.
Home players however (The Target market for Arranger manufactures) want something that they can just switch on and play, and if they wish to add other things, (Such as VSTs) a simple way to load and play them with minimal technical knowledge and effort. (Professionals who use arrangers live also want something that they can easily setup to suit the audience at the time)
Remember keyboards are designed for music players, not technology experts. (How do you explain to the layman that although you can load 100 or so VSTs, you can only use 3 or 4 high quality ones at the same time before the CPU/Ram gets overloaded)
Golden Rule
Music First, Technology Second

Bill