Why is the door locked to get inside seventh gym on fire red?

The door to the seventh gym in FireRed (the Cinnabar Island Gym) is locked because you need to defeat Blaine, the gym leader, first.

The lock is a visual representation of Blaine's challenge. You must prove yourself worthy by defeating him in a Pokemon battle before he grants you access to the gym.

It's a common mechanic in Pokemon games to use locked doors or barriers to signify the need to overcome a specific challenge before progressing. In this case, the challenge is defeating Blaine.