Why isn't CMOS data held in flash ram these days ?
It is, but not the same kind of flash ram as used in USB thumb drives.
USB thumb drive use a non-volatile technology that requires power to read/write, but the state of the bits stored is not lost when power is removed.
CMOS chips use a type of volatile memory that loses it memory when power is lost. The state of the CMOS memory is maintained using a small battery, so that settings and such are not lost when the computer is powered down. This is necessary because it allows for CMOS memory to be cleared on-demand by using a jumper or switch (effectively terminating power).
If the CMOS used the same type of flash memory as USB drives, and the memory became corrupted, or the system is unbootable because of incorrect settings, there would be now way of resetting it back to factory defaults without pysically replacing the memory chips.
Also, is there a reason why you're asking this? I'm going to go out on a limb here, as guess that whenever you power up your computer, you get a message similar to "CMOS Settings Bad", or "CMOS Date/Time Bad". If this is the case, then the battery that maintains your cmos settings is dead or dying, and needs to be replaced. Go to your nearest computer repair shop for more information on replacing the battery. Worst case senario is you will need to replace the entire motherboard...
