Because - and _ break text selection in existing systems you do not have control over, if you use those characters your ids will become harder to select.
Someone already made this point, and once again, "break" is completely undefined. It is not at all impossible or even difficult to select text with - or _, so what's "broken" exactly? At worst it takes one extra step to extend the default selection. These are such weird objections.
I work across multiple machines with different pointing devices (regular mouse, vertical mouse, touchpad), and have no issues double clicking to select a word. Dragging from the start of a word to the end can sometimes take multiple tries. I may miss the first letter. I may drag too far. The vertical mouse isn't great at holding a selection. It's not a huge deal, but it's an annoyance that I don't run into working with Stripe IDs.