The GHE is calculated by working out the effective/expected temperature of 'the surface' of a planet by just looking at how much radiation it absorb (i.e. incoming solar radiation minus the amount reflected) and plugging the number (240 W/m2 average for Earth) into the clever formula.
The 'surface' means 'whatever the sunshine hits first', which on Earth is mainly clouds; or all clouds in the case of Venus. So you are actually calculating the expected temperature of the clouds; the Alarmist trick is to compare/confuse that 'surface' (from the point of view of incoming radiation) with what we mere mortals consider to the the surface i.e. land and oceans, which are of course a lot warmer because of the gravito-thermal effect (unless you go to the top of Mount Everest, which is above the clouds and hence no GHE there either).
The clever formula works much better when applied to Mars or the Moon, which have little or no clouds. The actual surface temperatures are pretty close to the calculated effective temperatures (if you adjust for solar angle and night-time cooling; but there's also little or no atmosphere so you don't need to worry about warmth being transferred from Equator to Poles or from day side to night side).
Luckily, on Earth there are also places with little or no clouds; they are called deserts. A desert at the equator gets 1,361 W/m2 sunshine at midday. Divide that by the sq root of two to get average during a 12-hour day = 962 W/m2; halve that for the 12-hour night = 481 W/m2; then knock off one-fifth reflected = 385 W/m2.
Plug 385 W/m2 into the formula and you get an expected average temperature of 287K. Deserts have a large diurnal temperature range, just like The Moon (they cool down fast during the night because there is no cloud blanket), of (say) 40 degrees. A typical desert near the equator is just above freezing just before dawn and 40 degrees hotter in the early afternoon, so the average actual temperature is about 293K.
OK, that's six degrees warmer than expected and these are only back of an envelope calculations, but it's nowhere near the much vaunted 33 degree Greenhouse Effect, despite there being more CO2 above the Equator than anywhere else (the troposphere is much thicker over The Equator and The Tropics, and much thinner over The Poles).
The answer to the question in the title is of course "Because there are no bloody clouds over the desert, and the Greenhouse Effect on Venus or Earth is purely down to clouds and their altitude!", just in case you were wondering.
Debate Magazine
