If each solar farm consisted of one giant solar panel I'm sure we would see exactly the same problem. In reality building a solar farm involves redoing one simple step over and over again. It's much easier to scale up.

Offshore windfarms cost billions. A 10% overspend is quite significant.

Much better than a 300% overspend. Any renewable project that is going in that direction would be cancelled. Not only are renewables easier to build in small modules, but the staged build-out allows any problems (like, bad cost estimates) to be identified and fixed (or aborted).

The modular nature of renewables also allows them to be much more resilient in the face of component failure, or in face of construction errors (with the exception of common mode design failures). This means they can be installed with less skilled labor that makes more mistakes.

Renewable fields are akin to server farms w. large numbers of replaceable blades. Nuclear plants are like mainframes.