It’s A Good Thing Dragonball Z Villains Get Stronger In Small Increments (Comic)
The next villain always just happens to be stronger than the last, but not so strong that the fighters have absolutely no chance against him. It’s necessary to keep the tension, but it does seem kind of coincidental.
Comic Credit – JHall
For the most part I agree. There is no reason aside from plot that Raditz, Nappa, Vegeta, Friezas army or Frieza should have been anywhere compairable. Both Cell and Majin Boo required the Z teams super energy to become as strong as they did. If Goku hadn’t gone Super Sayian Freza would have easily won. Cell was actually too strong for them. They needed train in the time chamber to catch up.
So in the end it is some plot device that allows the Z team to level up quickly to fight the villain and win.