Because wind and solar etc have a few disadvantages, especially that they are not steady, sometimes the wind is strong, sometimes none, sometimes the sun shines

Because wind and solar etc have a few disadvantages, especially that they are not steady, sometimes the wind is strong, sometimes none, sometimes the sun shines, sometimes it doesn't (especially during the night). And power grids don't like energy like that, it can collapse the whole grid. So you have to buffer your energy, and that is expensive and not so easy to do. Nuclear or (if it works one day) fusion will give you a steady source of energy.