Power Rangers actually did a clever job of explaining why the Rangers couldn't just start off in the Zords. When the Rangers were first given their powers, Zordon laid down some serious ground rules, namely:

1) They were never allowed to attack anybody first.
2) They had to keep their identities a secret.
3) They could never escalate any conflict unless their enemies did so first.

As such, on every single episode, they had to fight the monster in an inconclusive manner until it finally entered its giant form and shot lasers out of its eyes.

The show's premise was taken largely from Voltron, which was, frankly, much better. (amended) Because of Voltron's popularity, the concept of the show seemed a bit stale (unless you were in elementary school, of course).

And it's still on the air today, albeit in different forms. Power Rangers series over the years include:

Mighty Morphin' Power Rangers
Mighty Morphin' Power Rangers plus the Green Ranger
Mighty Morphin' Power Rangers plus the White Ranger, who was the same guy
Mighty Morphin' Power Rangers with the new Rangers, with Rita and Zed
Mighty Morphin' Power Rangers where all the original Rangers are children except Billy.
The Mighty Morphin' Power Rangers movie, a minor box office children's hit for FOX.
Mighty Morphin' Power Rangers in space (all new characters)
Mighty Morphin' Power Rangers where all the Zords are emergency vehicles. "Rescue Force" or something.

I watched this when I was eight, and it's still on. Mein gott.

And that's not even bringing up the villain that looked like Gwen Stefani and always wanted to go shopping.

asterphage points out that the Voltron statement is ludicrous, and offers excellent evidence to prove it. Bravo, sir. It is changed. See sentai for some quality information. I do stand by the fact that Voltron is better than Power Rangers... but no one is arguing.