Why is cross chaining 'bad', but 1x is OK?
One key difference is the design of the chainring. A traditional ring is made to allow shifting, which means there’s specially shaped teeth and protrusions that will pick up the chain and help lift it in to the next gear.

Cross chaining with traditional rings will put the chain in the angular region where it ‘thinks’ you want to shift (ie. these mechanisms will start hitting the chain plates).

On a 1x system a narrow wide ring is typically used, or a similar device, that meshes more completely with the whole chain link. Since it doesn’t have to worry about shifting, it’s only worried about retention, so it can ease the chain into a consistent position from a wide variety of angles.

As for the rear derailleur, they’re designed to rectify the angle as well, with SRAM opting for a more eccentric pulley position relative to the pivot, and Shimano using differently shaped parallelograms. Both these systems keep the pulley a consistent distance away from the sprockets, so the whole system can accommodate wider chain angles.