I guess Sonos simply chose to sell the charger power adapter separately, as not everyone required one… but it’s easy enough to also add it to the shopping basket before checkout. See below link:
https://www.sonos.com/en-us/shop/roam-power-adapter

It's a big thing in the green movement to reduce redundant charging devices, and Sonos has recently put a large effort towards greening their image.  This choice to not include a charger is in line with their new packaging, lowered standby power usage and other efforts to lessen environmental impact.