And why is the minimum ceiling for a visual operation on an aerodrome 1500', or 1000' Special VFR?
You are confusing an IFR flight manoeuvring with visual refernce to the runway / environs and a VFR flight which is something totally different.
The ceiling of 1500ft for a VFR flight in a control zone come about as a result of two other ICAO rules;
1. minimum cruise height of 500ft AGL and
2. requirement for a VFR flight in such a case to be 1000ft vertically from cloud.
i.e. if the ceiling is 1400ft you are not going to be able to comply with the above ICAO rules.
If I make an ILS approach with a DH of 200ft and an RVR of 550m, the bit from 200ft to touchdown will be a visual manoeuvre. With circling there is simply a lot more manoeuvring before touchdown.