I didn't decide that, the designers did.
Then within the current rules there are options for that player through multiclassing. What entitles you to say that every class should be able to combat every shortcoming? That kind of attitude of "there's always a counter" led to 3E orb spells that bypassed SR because someone thought spellcasters "needed" to get around that one thing that put a speedbump in their way.
First off, I edited my posts a bit to remove some of "you" pronouns, which can add a needlessly hostile tone.
Regarding the 3e orb spells, 3e had the right idea. Spell resistance blunted a lot of the wizard's heavy-hitting evocation spells, so they created an option that provided some less heavy-hitting conjuration spells that bypassed SR. That's giving players options with give-and-take--do you pick the big nuke, or go for a single-target spell that bypasses a pesky defense? Now, they screwed up that approach with some other conjuration spells like Cone of Flame, which were just as heavy-hitting as the evocation spells, but that's a flaw in the execution, not the idea.
Contrast that with 4e's approach to SR, or DR, or crit-immunity, or the undead's immunity to mind-affecting effects...They took them away, leading to fewer interesting options for both attack powers and monsters to use them against. Undead are just lame radiant-vulenrable immunity-defecient creatures.
However, while monsters lack puissant defenses in 4e, they do have potent and interesting attacks, many of which take away movement. This should lead to some interesting choices for players mulling over their power options: should I take a ranged power just to have something in arsenal for when I'm glued down? By denying them this option, the end result is the player thinking "well, screwed again", rather than "darn, knew I should've picked the ranged attack just in case.."
The designers decided that the focus of rogues and rangers had room for both. Why another striker, the barbarian, doesn't have similar options built in I'm not sure.
The designers simply don't know best a lot of the time. Constant updates to the core rules and patch feats like Weapon/Implement Expertise and Distant Advantage are good proof of that--but at least it shows they can address their glitches. And sometimes they're intentionally leaving a hole to be filled later with a splat book. Bards, for instance, were turned into arcane archers with Arcane Power.