Because the other ones are created with magic or PCells in Klayout after the parameters you specify in for example a SPICE netlist. Like the width and length of the gate, etc. The other ones probably have a slightly unique shape which is harder to automate and since not many people need them there Just hasnt been demand for it to have it in magic (my explaination at least)

More than just a unique shape (which is true), each of these layouts has a unique characterized model that is independent from the model for the underlying device type. If you depart from the exact layout, then you depart from the SPICE model accuracy.