They are more expensive but still cost effective (i.e. cheaper than burning fossil fuels or nuclear power). And winds tend to be more predictable and stable on the ocean. So, the capacity factor is better. And you are right that off shore wind is a lot less controversial. More importantly, it's way less hassle and risk. There are no NIMBY's blocking it. There are no complex land ownership issues to resolve. Magic wands to make all that go away sadly don't exist. So, there's very little land where you can just decide to plop down a few hundred wind mills without getting bogged down in years of bickering with the locals, sorting out all sorts of complex issues, etc.

Also, out on the ocean there are no roads to navigate so moving big/heavy things around is a lot easier to do. All you need is ships. Transporting a blade that is over a 100m long over land is hard and expensive. The infrastructure just isn't ready for that. Off shore wind mills can be bigger than onshore wind mills for this reason. Floating offshore wind is now also becoming a thing. This will unlock areas further away from the coast.

Mass producing, mostly pre-assembled wind mills and shipping them out is a scalable business. Companies have been doing this for a few years now and it's working. They are getting good at it. They are making incremental improvements and learning ways to improve things. This learning effect is very real. There's no shortage of suitable spots to place these things. It's more a matter of how many GW do you need and how expensive is that going to be?