The costs to repair submarine fiber is high regardless of whether they're buried or not; they're already dragged up when they're cut by things like anchors. It

The costs to repair submarine fiber is high regardless of whether they're buried or not; they're already dragged up when they're cut by things like anchors. It's more about the cost to bury the cable at a particular depth vs. the risk of it being cut (and expected cost to repair and outage cost). There are guidelines to determine how deep to bury cable based on risk like the Cable Burial Risk Assessment. In shallow waters like the Baltic Sea, communications cables are buried to protect against things like fishing boats snagging them. For example, the C-Lion1 cable that was cut was buried a meter down. In deeper seas where the risk of accidental cuts is near zero, there is little benefit to burying them.