Because Epic (and Elemental and Event) categories give 3*, 4* and 5* heroes.
(Daily Summons gives 1*, 2*, 3* and low-level Troops, however in my experience, 3* in Daily is rare.)
Be aware that the game also gives a free-to-play option in the Training Camps at lvl 12 (3*), lvl 13 (mostly 3* with some 4*) and finally lvl 20 (3-5*). Everyone has slightly different experiences as regards what you get (which hero, etc), but this option is available to every player and costs no diamonds.