With the way Sentai robots combine, all articulation would do is cause parts to break off and possible injure kids. I'm happy with them making DXs that can stand (Gobuster Beet is kinda pushing it) and having the Miniplas for articulation.

Because making secure docking points would be to hard?

I think its a preference rather than laziness. Or they wouldnt make the miniplas so damned poseable in comparison.

The Daibouken is the rare exception that includes brilliant articulation and doesn't suffer from it. And it's even more rare that the articulation gets carried over in all of it's combined forms.