A lot of times they aren't even real to begin with.
People assume that economies of scale keep going up as long as scale keeps going up, but that's almost never true. They typically have diminishing returns or thresholds past which the unit cost stops going down. If you want to build solar panels you have to build a factory. If the factory can produce a million solar panels a year and you only want 10 solar panels you still have to build the entire factory. It's more efficient to build a million than 10.
But if people want a billion solar panels a year then you need a thousand factories, and one bigger factory isn't materially different than having two factories across the street from each other, so there's no real advantage to having them all operated by the same entity. Moreover, even if you only need 10 solar panels, you can get them from any of the thousand factories that each make a million a year. You're not losing the economies of scale by having many sellers and many buyers.
Meanwhile centralization often incurs additional costs. You already identified several, but another big one is land. Individual homeowners each have a roof or balcony wall that was otherwise going to have nothing on it. A centralized solar farm is more often going to have to pay for space.
Centralization is usually pushed by someone trying to monopolize something.