Why do I need to give my credit card info to get a free trial?
You don't need to if you want to just try it by redeeming directly one of the two free to play games
https://stadia.google.com/games/destiny-2
https://stadia.google.com/games/super-bomberman-r-online
But doing so you will miss the month of Stadia Pro trial, to have the free month of Stadia pro credit card info are mandatory.