All I can really think of is for the rare time you need to texture a 300ml jug?
And if your using a commercial machine with a 20-something litre boiler at 1.something bar and a 4 hole steam tip, that *might* just be too much for some people... (I still use the VBM steam wand at around 60-70% power when I am only making one drink, otherwise I get too thick microfoam (maybe closer to minifoam?)) and even set like this it takes 8 or 9 seconds...
Also, correct me if I am wrong, but its probably also for look. Most companies try to keep their machines "traditional" looking. And tap valves have been around alot longer than solenoids, so they just kept on with the knobs as it was what the people buying the machines were used to and wanted.
I also think its a bit easier to change the O rings in a tap than replace a solenoid/switch/electrical parts etc... (not to mention find out which is the problem if something does go wrong.)
Any other theories?