The better wind conditions help, but offshore wind is really expensive. Transport and install alone for a big offshore turbine can easily cost EUR 1-2M. Add to that undersea cables, higher maintenance costs (salt water, difficult access) and it is clear why onshore wind (if possible) is still the preferred choice. Nonetheless, mainly due to the up-scaling and abundance of available ocean space offshore wind has taken off in the last few years.

Offshore siting also allows for much larger wind turbines, which are needed to economically exploit the higher quality winds given the extra expense of offshore construction and maintenance.

Onshore turbines are size constrained by transportation limits. Waters-edge construction and transport by sea allows for larger elements.

There are other advantages. Urban centers are often a lot closer to the sea than they are to the large open spaces suitable for wind.

Yes. Land based ones are often on plains or ridges. Steady and unimpeded wind is why aerial and offshore have better EROI.

Do land-based ones compete with agricultural usage?

No. In fact, they make marginal agricultural operations economically viable by providing a second source of income to the farmer/land owner.

We have huge wind farms here in Minnesota and Iowa. They're in the middle of crop fields. No problem at all.