The estimation method is determined by the scale of the endogenous variables. The model is estimated conditioned on the observed exogenous variables. Their means, variances, and covariances are not parameters in the model. When you bring these parameters into the model, you make distributional assumptions about them and treat them as endogenous variables.
If you don't mention the means, variances, or covariances of the observed exogenous variables in the MODEL command, you make no distributional assumptions about them. And you don't do that. This is in line with regression analysis.