This is due to the fact that it is non-volatile memory. You can store the chip forever and the data will always be there. Besides, the data is very secure. The BIOS is stored on ROM because the user cannot disrupt the information. This is basically a blank ROM chip that can be written to, but only once.
Some companies use special machinery to write PROMs for special purposes.
This is just like PROM, except that you can erase the ROM by shining a special ultra-violet light into a sensor atop the ROM chip for a certain amount of time. Doing this wipes the data out, allowing it to be rewritten. This ROM can be rewritten through the use of a special software program. ROM is slower than RAM, which is why some try to shadow it to increase speed. It is volatile memory, meaning all data is lost when power is turned off.