Halliday & Matthiessen (2014: 304):
In Old English, the structure was he said/thought that: he was not hungry, with that as a demonstrative in the ‘verbal’ or ‘mental’ clause ‘pointing’ to the clause representing the reported content of saying or sensing (see e.g. Hopper & Traugott, 1993). This demonstrative came to be reanalysed as a structural conjunction introducing the reported clause; but the reported clause itself remained outside the structure of the reporting clause – it has not been incorporated through downranking (in contrast with ‘fact’ clauses). Thus we would not expect to find such reported clauses serving as the Subject of a ‘receptive’ ‘verbal’ or ‘mental’ clause; for example, that he was not hungry was said/thought by him is highly unlikely.*
* But note instances like It was said that Feynman played the bongos, where the projected clause looks like the embedded postposed Subject of was said. Note also agnate instances like Feynman was said to have played the bongos.