This is a strong assumption. How can we have evidence for that? We do not understand climate enough to make such an assertion.
This is the answer to your question. Enough people see that strong assertions for man-made climate change are not necessarily backed up. Hence the controversy.
> but furthermore I don't see why we shouldn't transition to renewable energy asap anyway
Because that comes with it's own problems? For instance, around here there are a lot of wind wheels, these machines create problems. It's not just "energy for free". For example one problem is they kill a massive amount of insects.
There are also people advocating for nuclear energy, improved ways of providing nuclear energy. Maybe they are right?
> Even if climate change would be a hoax (which it is not), then why would it be a bad thing to make changes to prevent it from happening anyway?
This is a good question I asked myself many times as well. I don't know.
One part of the answer could be priorities. We can't do everything at once. SO what comes first?