You're more frequently entering the primary user password (screen unlocking, logins, sudo...) so there are substantially more opportunities to observe it either

You're more frequently entering the primary user password (screen unlocking, logins, sudo...) so there are substantially more opportunities to observe it either visually like over the shoulder or from camera footage, or through a compromised machine fully online. Generally the FDE is unlocked in an offline and minimally functional state, and being a one-time entry @boot kind of thing it's far more practical to secure with things like a temporarily connected crypto hardware dug out of your bag or somesuch. There's clearly some value in making them different.