Why Keep Forming Voltron?

Voltron was once a single, sentient robotic Mecha. Depending on the original Japanese cartoon where an unnamed "space goddess", or the American adaptation where the evil witch Haggar, cast a spell that separated Voltron into five robotic parts.

GoLion's story is set in 1999, when the planet Altea had already been subdued and enslaved by the Galra Empire. Five space pilots return to Earth to find the planet annihilated by thermonuclear war. Eventually, the explorers are captured and enslaved, and forced to fight for their lives in Emperor Daibazaal's arena. The young pilots escape and eventually land on the planet Altea, where they discover the secret of the mighty sentient robot GoLion, the only one powerful enough to defeat Emperor Daibazaal's forces. Thousands of years ago, GoLion was an arrogant robot who, after defeating several beastmen, tried to challenge the Goddess of the Universe to battle, but failed. To teach him humility, the goddess separated him into five pieces in the form of five lion robots that sailed through space and crash-landed on Altea, to lay in wait for those who would one day reawaken him to fight evil once again.

Once split up, the five parts (lions or vehicles depending on version) are needed to be piloted to form Voltron. Once formed, Voltron has a time limit (5 Minutes in one episode, the lowest it was) before it gets split up. The limit is eventually ignored.

The fight rages between Voltron and the robeast, but it is perilously approaching Voltron's 5-minute limitation. Voltron finally gains the upper hand and, forming his Blazing Sword to finish the job, destroys it with gusto.

"But remember he has only 5 minutes of stored nuclear power. He's our last line of defense."
—Commodore Steele notes Voltron's limitation

And Voltron can't stay formed without the five parts being actively piloted. If they aren't in the lions/vehicles, then Voltron splits up.

Additionally, the Lions can heal or self-repair when separated, but not when joined together. They are also often used to scout different areas, as fully capable weaponized space ships. Keeping them together as Voltron the entire time would be a waste of resources.