Why do anti-ice systems on airplanes with automatic ice detectors still need to be manually selected on?
Because most of the time the system has to be on even when there isn't detectable ice accumulation.  Because of what I like call &quot;technical icing conditions&quot; but which are just called &quot;icing conditions&quot;:
ICING CONDITIONS:

TAT (total air temperature - accounts for temperature rise from compresibilty) less than 10C

Visible moisture (cloud, haze, anything that is water)

SAT above -40C


You also have to have WAI on for takeoffs when OAT is less than +5C with visible moisture (including visibility under 1 mile) regardless of whether there is actual ice formation (water streaming back on the wing on takeoff can freeze upon rotation due to the temperature drop from the low pressure over the wing).
When you get the wings deiced, you still have to have the WAI on at the start of the takeoff roll if departing in technical (or actual) icing.  You don't want to cook the thick type IV holdover fluid, so you leave WAI on selection the very last thing as you pull out to line up for the TO roll (the CRJ200 has an evaporative WAI system which meant the leading edges get VERY hot, about 105C, which could cook the fluid and make it crusty).
When flying, the only time you can leave the WAI off when in technical icing conditions is
above some speed (on the CRJs it was 230kt), unless you get an ICE caution from the ice detector.
The result is a system that automatically turns the WAI on when ice is detected would only cover about, oh, maybe 10 - 20% of the time you are legally required to have WAI on.
Cowls are similar, but have to be on even more often because there isn't the 230kt threshold for actual accumulations only. They have to be on all the time you are on the ground less than +10C with visible moisture (freezing around cowl inlets from velocity temperature drop).
Without the 230kt exclusion the wings have, you have to have the cowls on pretty much all the time you are in cloud, actual icing or not, so you are turning them on and off quite a lot if climbing/descending through layers (some pilots may just leave them on), whereas outside arrivals and departures the wings are only on when you get actual ice.
So you could automate the system like that, if you could figure out how to make the system reliably detect &quot;visible moisture&quot; outside somehow (probably possible, but very expensive).   Otherwise, the decision to turn it on is a crew decision based on the view outside, probably 80% of the time.