They have solar + batteries, so they have to convert the sun into DC, hopefully directly supply that DC to their batteries, then convert that DC to AC power, then convert that AC power back to DC. Converting AC to DC or vise versa is a reasonably lossy process, so not doing double the conversion is even better.

Also some less than ideal setups convert solar directly to AC, so in order to charge your batteries you have to convert it back to DC.