Look at all the guys wearing jackets under their uniform.
We've got pitchers...
Even the manager was cold!
The other possibility for all the jackets is that it was hot. Yes hot, because back in the day these rubber jackets were worn to sweat off a few pounds. The Cubs were aging and maybe they put on a few extra pounds over the winter and wanted to shed them. Hot day plus jacket equals weight loss.
Hot or cold, you don't see the jackets any more.