Why can't I merge these text layers?

I figured it out! I didn't need the text to be editable any more. I just wanted to add effects (drop shadow, bevel, etc.) to all of it at once.

What I did was to convert all the text into one "smart object." Then I could add my effects to the entire group at once.

Assuming what you want to do is take two or more text layers and put all the text on a single layer. So that the text is still editable?

You can't do this directly unfortunately because each text layer can have only a single entry of either Point Text or Paragraph Text. You can copy and paste the type itself from layer to layer, but each layer can only contain one text "object."

However you can change the font and attributes of mutliple layers by highlighting them all in the layers panel, and changing attributes.

You can't merge text layers, but if you target them together (hold down shift or cmd key and click on the layers) then the  Character Panel and/or Paragraph Panel will edit all.