Their markets are different too. Jet2 Holidays are recognised to be of good quality, fully inclusive, but at a higher price than just the base components.
On the other hand, EZY sell you a flight, then start adding extras - baggage, seating, hotels, car hire, etc. With EZY they first tell you the base price of the flight, with Jet2 you are given a holiday price.
Both have their niche, and both are good at what they do.