This isn't the case - I am not aware of any CMIP6 model (the latest generation used in the current IPCC report) that doesn't include clouds.
Clouds are definitely complicated, as they are smaller than a typical model gridbox. This means that there are lots of uncertainties about how to include them in climate models. Their response to a changing climate is one of the leading uncertainties in our understanding of past and future climate.
As you say, it would be difficult to simulate the climate if you didn't include clouds, and so models absolutely include them.