It's because of what the Modchips counter: The copy protection mechanism.
Originally Posted by Quad5Ny
The boot ROM (aka "BIOS") of the Playstation 2 has got nothing to do with preventing copies of original Playstation software discs from being run.
Modchips interfere with the communications between the drive MECHAnics CONtroller and the DSP chip, to trick the MECHACON into thinking that the disc inserted has a valid security code where burned discs will lack.
Anyway, the OSD of the Playstation 2 has a mechanism for booting updates from a memory card, so we don't have to replace the contents of the boot ROM chip on the mainboard. Unless it's a late SCPH-90000 series model, which has that mechanism permanently disabled.
SCPH-10000 S. MINOKAMO v1.01 (Defunct)
SCPH-10000 S. KISARAZU v1.00 (faulty)
SCPH-15000 S. KOHDA (With warranty seal!) :D
DESR-5100 S. EMCS