well that's because 'balancing the martial' by giving them spells would fundamentally make them no longer a martial, or at least change the execution of them so far away from the point of a martial that we might as well have just picked a spellcaster in the first place.
"Oh yes sir, we can fix up your clockwork clock that's getting a bit slow, we'll start by ripping out all those useless gears and cogs and then replace them with this digital display"
we want the martial balanced as a martial, not as another spellcaster in disguise.
Unfortunately, nobody can agree on the design of what that would look like, even as WotC seems fairly committed to devoting much of their design to new spells than new mechanics for subclasses or feats.
Like I get it, spells are easy to design for, you don't need to tie them into any other effect. A subclass has to have a theme, and they'd rather put passive abilities there as opposed to active ones. But when that design is walled off from classes unless they have an explicit "magic coupon" to ride that ride, it's pretty damning.
I mean, 9 years of 5e and what have martials gotten? More subclasses, sure, but outside of that? A couple of alternate class features in Tasha's, a very limited selection of new Feats (because
optional!*), and only now, weapon masteries (with almost no new weapons outside of the PHB).
Meanwhile, casters get all that
and new spells in just about every book. Why is the lion's share of design aimed at casters? Because spells are easy to design, they don't need to be tied to any other bespoke mechanic, they don't need to fit a theme with other spells, and if you can cast spells, all it takes is putting it on your spell list.
Yeah, someone will mention I ignored the new Battlemaster maneuvers. Sorry, toys for
one subclass of
one class doesn't really help other martials very much. Plus, I mean, it's even easier to design maneuvers than spells! We could have a hundred maneuvers by now easily!
Mini-rant: also, can I just say, the idea of walling off certain content as
optional feels like such a copout.
Everything is optional in D&D! A DM can decide races, classes, subclasses, weapons, even spells don't fit the theme of their campaign! The only thing labeling something as optional does is say "yeah so, we didn't really balance this well, so use at your own risk", despite the fact that public play uses Feats, and their adventures have magic items in them,
and they keep making new magic items!
It's especially heinous when their optional rule comes with a safeguard (attunement). Wait, if it's optional to even use magic items, why are you suggesting a limit on them?
Mini-rant 2: and why do we even use attunement anyways? Because WotC suggested it? What other version of D&D has limited how many items you could have (outside of item slots, I suppose). If the DM controls the magic item acquisition, why even have attunement as a thing?
And nobody has a problem with this, not even people who profess a love of the old school style of D&D, which had no such mechanic either! It's a jarring change to one of the most fundamental and awesome parts of Dungeons and Dragons, to the point it almost* doesn't feel like D&D, yet I never hear anyone gripe about it!
*I say almost because there are a few magic items that actually do something extra when attuned, which really makes you feel like you've "mastered" the magic item. Kind of like being able to resist the domination of a sentient item or artifact, and getting to use it's full power. But these items are rare, and the only one I can think of off hand is the berserking axe from Hidden Shrine of Tamoachan, which, when attuned, gives you....5 more hit points. Wow.