innerdude
Legend
Maybe because they are, as you say, full casters. Spells are a major component of their class, it's an area where they are focused. If you want to mix it up more in combat and cast a spell maybe every other round then you have your paladins and rangers.
Really speaking, it's the same with the other classes, if you make a fighter focused on archery, you're probably going to be shooting a bow every round, because that is your focus, it's what you're best at and every round you're not shooting your bow is probably going to feel like a wasted round. Sometimes, you may switch to a sword to engage in melee, just like a cleric may switch to a mace to engage in melee if it calls for it, but I wouldn't be surprised if people who miss with their melee attack start to wonder if their +2 bonus from archery would have meant they had enough to hit.
Or you use a system that assumes characters are good at more than one thing. My current Savage Worlds character would be considered a "full caster" in relative D&D terms, but also has the highest archery skill in the party and several spells that are combat-oriented but don't do direct damage. The battlefield options available to me are massively more varied than simply "pew pew" magic.
Do I hold this round to get my +2 aim bonus and almost guarantee an extra damage die when I fire? Do I use my legerdemain spell to perform a dirty trick from 30 feet away on the opponent my hand-to-hand fighter is engaged with? Do I cast a damage bonus spell to give +2/+4 damage to the dwarf? Do I draw my short sword, roll into melee next to the fighter and use a defensive maneuver, giving the fighter a gang-up bonus? These are all viable, and depending on the situation, highly effective potential options.
But this kind of assumed broad competence is generally only possible in a classless, skill-based system. I don't know about 5e, but in 3e you'd have to be a multiclassed ranger/sorcerer of about level 8 and in all likelihood have a level of the Arcane Archer prestige class to do what I'm doing in Savage Worlds at the functional equivalent of level 2.
Now of course, I always had the option of focusing strictly on spellcasting. I could have totally stat dumped my fighting and shooting skills, and just pumped spellcasting to the max. But Savage Worlds wasn't meant to be played that way; it assumes as a system that characters will be broadly competent in a variety of skills. It's not to say I'd be "playing it wrong" if I min/maxed my spellcasting, but I certainly wouldn't be playing to Savage Worlds' strengths as a system and would certainly be cutting myself out on a great deal of the potential fun.
But on a related note --- There are no "cantrips" per se in Savage Worlds. Every spell cast has a real resource cost in terms of available power pool, and casting is not an automatically assured "thing." And to me, magic never feels like a "substitute" for something mundane.