Android boot architecture compared to desktop boot process
To put it simply, it is not designed for users to "boot from Memory card" as you put it.  It is an embedded device and is designed as such.  Android devices are typically not meant for the end user to install software via external media.  They are specifically designed for additional software (not new OSes) to be loaded via some network connection.  The initial boot parameters are really going to be dependent on the product.