Up to now wind turbines offshore have each been built as an individual column with the turbine on top but as the water gets deeper that becomes more and more difficult structurally. So more complex structures are likely to be needed. Oil rigs have coped with this by being multi-legged but there are less of them. This is all new for offshore wind farms, there may been solutions that worked for the oil industry but are they economical for the wind farms? Perhaps they could be made as a massive floating network, tied by cable to the sea bed and each other but allowances have to be made for the surface swell, stability, etc.
It is easy to say, ha! this has all been done before by the oil industry but engineering is always about compromise, the set of compromises that were acceptable for an oil rig may not be the same as those for wind farms (a benefit is that the wind farms won't be permanently manned unlike most oil rigs).
Combined with the additional cost of laying miles of undersea cable, apologies if I didn't make it clear I was talking about the cost of laying it by the way, makes the whole thing extremely expensive to do. And lots of expense needs a big return on capital.