The downside of cable burial is that repair is harder.
At the end of the day, buring cable throughout it's way is very expensive while damaging it at one spot, even if it's buried, is not that hard or expensive.
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.