Why doesn’t this come with USB?

Because adding USB input would increase the cost (both to Kanto and to you), and Kanto’s market research probably showed them the majority of buyers would be using this in their living room or at a desktop computer (many desktop motherboards have optical out), not at their laptop.

To connect digitally to your computer you will need a USB to S/PDIF converter. This one will work fine:

https://www.amazon.com/LiNKFOR-Coaxial-Headphone-Converter-Optical/dp/B07SR7BYD6

Or you could step up to this one which can handle some additional formats and sample rates (not necessarily for most people).

https://www.amazon.com/Douk-Audio-Converter-Interface-PCM192Khz/dp/B085XPRSGM