Some do it seems, from the Blakley data sheet:Why don't building site transformers have MCB's on the fly leads to what they are supplying?
Overcurrent Protection
Input MCBs are Type “D” and output MCBs are Type “C”, to BS EN 60898.
All 32A sockets are individually protected by 32A DP MCBs.
Lighting circuits without RCD are protected by 10A or 16A DP MCB, as detailed above.
Taken from this product:
Site Power Transformer with MCB Protection | Blakley Electrics
Site Power Transformer with MCB Protection from Blakley Electrics, designers, manufacturers and distributors of high performance Power and Lighting products.
www.blakley.co.uk