in the discovery F746 board, the I/O pins are not accessible?
Most everything is committed to a specific purpose. The Arduino shield socket is best means of expansion / exposure of available signals.
Use the NUCLEO-144 if you need a breakout board.
Discovery board implement built-in feature of components requirering high speed signals which wouldn't survive well through a connector (DSI, USB, SDRAM, QSPI, uSD), left pins are the one going through connectors, and as Clive said, Arduino is the prime access to some GPIOs. On newer discovery kits, you may find more GPIO accessible through PMOD/STMod+ connectors (which are feature rich GPIOs), making the discovery kits a bit closer to Nucleo's.
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