Do you really think a nuclear plant is just a building?

Wow.

Anyway, nice talking with you.

You're not completely wrong but neither is the person you're replying to. While the raw materials of construction may have an established supply chain, NPPs are unique in at least two ways:

1. Each has a somewhat different engineering design to account for conditions of where it's built; and
2. Since the designs differ, the construction process necessarily differs and, due to uniqueness, is inherently more expensive and complicated than just building something off-the-shelf or standardized like a house or office building (or, relevant here, a wind farm).

Raw materials is only part of the supply chain: there's construction (as you mentioned), but also engineering and design.

The expense of NPPs, including going over-budget and having to adjust engineering designs for new regulations, is largely because NPPs are regulated to "internalize" their externalities. Whereas a coal plant is allowed to pollute in gathering the raw materials, is allowed to pollute in producing electricity, and is allowed to pollute in disposal, and has weak safety standards overall, NPPs must be mostly self-contained and over-engineered for safety. If coal plants had to control all of their pollution, be earthquake resistant, be airplane-hijacking resistant, etc they would also routinely be over-budget and have delays, and have unique designs for each plant. Now, there is something like a plateau here, where at some point we will have decided on a fixed set of regulations, and common design features can be identified and re-used more than they are now, and therefore NPPs could become less expensive. But we aren't there yet. Comparatively, we do have a practically fixed set of regulations and common design features for much of the renewable sources.

Currently, other renewables get to benefit from existing supply chains where NPPs can't really, but it doesn't have to remain that way, and there's reason to believe it will remain that way.