Because when it concerns volcanic ash...VFR rules don't work. You aren't just trying to stay out of the "cloud" and you can be VFR for days and still have enough ash in the air to stall your engines (if you are a jet).
Ash builds up over time on the engine...so even if you can't see it...its slowly building up in your engine...and grinding the little pieces down.
There is also the little problem of abrasiveness. Even if there is just a little bit of it in the air...it's abrasive. When it hits the cockpit window, it scours it (almost looks like a frosted up window). Then the pilot can't see whether its VFR or IFR. You want to fly with a blind pilot...go ahead...
Nah, I am not buying it.
If you have clear visibility there isn’t enough ash to cause any problems. Sure you might have some ash in the air (dust) but that is just routine maintenance issues.
I flew over to Mount Saint Helen’s just after it blew and it was easy to avoid the ash. And no, there was no abrasion on the prop or the windshield.