The crutch is the problem. Choices and consequences are important. The more feats on items, the more they get ignored at level ups because the crutch is often good enough to get by. To dabble one should pick the feat not get it free whenever they want it.
I don't think that holds true as a general rule, it heavily depends on what feats and how they are implemented.
In the case of saving throw DCs, there is not really a just good enough scenario. The more you invest, the better your spells are.
Take the apprentice staffs for example, they are widely available as common drops and even purchaseable from vendors.
Yet most wizards and sorcerer will pick up spell focus + greater spell focus for three schools of magic they want to specialize in,
because just spell focus (and often even greater spell focus due to the bloated saves, but that is another topic entirely) does not cut it.
This holds even more true to warlock due to their inherently lower spell level and thus DCs.
If you want to use them at least somewhat reliably, you have to heavily invest into charisma and take invocation focus and greater invocation focus.
Another factor is item rarity. Take the monks training cowl for example, it has a negligible downside of slightly lowering your spot
and gives one of the most important feats for any character that wants to do combat, blind-fight.
But due to its rarity, people tend to not build around. Who knows when and if you even gonna get your hands on one, so everyone just takes the feat.
Also a big point is, what other gear is available for the slot and is it worth skimping out on the feat.
Another example here: the hexbands a very common drop for the hexblade.
It grants the most important feat to increase your curse DC (curse focus 2-6 increased DC depending on your hexblade level) at no downside.
I know not a single hexblade who uses curses and does not pick up the feat.
If you look at what other options are available, for example enchanted steel plated ancient dire rhino gloves (+1 uni save, +4 discipline, +4 parry, +3 antagonize)
That is a lot more value then you can get out of a single feat.
A bad offender against all these metrics and where i would agree with you is for example moonfriend.
It grants a feat that every class with turn undead wants (extra turning),
at no downside,
in a slot with not to many high value options (ring)
is very common and cheap.
In contrast,
granting invocation focus ( a mildly useful feat for a warlock that does not specialize in DCs in the first place),
with a downside,
in a slot where there are other great options,
being somewhat rare.
Now that really would not be a big deal.
(FYI i do not support suggestions of the OP, giving invocation focus AND spell pen on one item, is way over the top.
Greater spell pen/invocation focus should be reserved for characters to invested at level ups)