Hybrid classes are really the only reason, but yes, the ability to hit RES is far more valuable to physical primary units than the ability to hit DEF, and is compounded by the fact that ranged physical damage (1-2 thrown weapons or bows), even against squishies, is likely an inferior option than magic.
Fire Emblem's base design space isn't really accommodating of the split, so you have to look at designing around it. A skill system offers one simple (but maybe not 100% effective solution) - Allow hybrid mage/physicals access to a simplified form of Ignis, wherein they can gain a bonus fractional of their MAG stat to their strength. In situations of WTA or targetting a low DEF unit, this may be enough.
Similarly, you could supplement that with a 'Spellbreaker' type ability that gives WTA with any physical weapon versus tomes, and you'd make hybrids into caster-killers.
Also as @Agro pointed out, increasing the prevalance of magic swords/lances/axes etc. also herlps.
Of course, in GBAFE, where design space is massively constrained by technical limitations, the waters get muddier.