Probably because playing a tackle means more money. Its more of business decision than just switching positions for the sake of the team. I know the Eagles had in issue with one of their guards last year refusing to play tackle and wanted more money. He eventually said fukk it though and played tackle I think.
Maybe in the long run for somebody who was a guard, and that position pays more than tackle, and then they move him to tackle permanently, and then try to pay him like a tackle, but Zach is in the middle of his latest deal and he is only playing tackle while collins is out. He is not getting any less money for playing tackle for a few games.
Like I said, you turn on any football game on sunday and in the first 5 minutes you going to hear about how some random team has shifted the line around. If Zach doesn't want to do it that's his business, but it goes to show you what type of work environment that the cowboys have that Zach even has the option to not do it in the first place.