Fwiw not *every* game lets you but SP items.
Some series are purposely stingy on "resource items" specifically because these tend to be more important due to active skills being stronger than regular ones.
And then there's SMT5 where this is never a problem
or SMT4 where it's a problem in the early game until it isn't
I believe Persona 5 offers a variety of renewal avenues too actually, Royal even more so.
Game design! It varies.
It's definitely a game-by-game case to the point trying to compare it to different games can obfuscate things. Often times it comes to just asking basic question in terms of the game itself. So, going back to Pokemon:
Main Question: Should the PP restoring Ether items be made more widely available, notably sold in Marts?
Follow-Up Question 1: Is PP a resource which needs close managing?
Answer: A bit of yes but mostly no. PP is very much a thing you need to keep aware of, BUT there's so many things which the game does to keep PP available for most of your playthrough you don't really need to think about it.
Follow-Up Question 2: Is PP difficult to restore?
Answer: No, there's plenty of healing spots (notably Pokemon Centers) which completely restores a Pokemon's PP (and HP).
Follow-Up Question 3: Would the game balance be effected if Ethers were made sellable?
Answer: For the most part no, because the game makes sure you're never low on PP, that also means Ethers would have little impact for most players.
Follow-Up Question 4: If made sellable, what scenario(s) would an Ether be considered in? What amount of work takes getting into that/those scenario(s)? How would that/those scenario(s) be affected with Ethers being more available?
Answer: The most obvious two scenarios would be in a non-competitive tough battle with a low PP Move (which are are usually powerful Moves) and being deep within a location you don't want to leave but are battling a lot of Pokemon. For the "tough battle", its fairly easy to get into but usually such battles you know are coming up so have plenty of time to prepare for them. Able to restore uses of your Pokemon most powerful Moves would make such battles easier, but because those battles you should have time to plan for you should also have back-ups and alternative options so honestly the difficulty doesn't really change all that much. As for being "deep within a location", that's very much a scenario you need to go out of your way to setup, not just for finding the location but also battling so many Pokemon (most which would likely have to be Wild for you to get so low on PP). If the purpose of you being in the location is just to explore, you should just be avoiding and running away from battles; do your exploring and get out. If you're using the location to train in, in that isolated case than yes Ethers would be helpful in keeping you there longer. However post game usually gives quite a few places to train, some with healing spots close by, that the question becomes is it worth training in that spot if it's going to take so much resources to stay there. Even if the other locations closer to a healing spot isn't as great, having a healing spot right there is a heavy pro as when you need more PP it's a quick trip and back to training; the time and resources saved may tip the scales for the convenience.
Main Question Re-stated: Would the game be negatively affected if the PP restoring Ether items be made more widely available, notably sold in Marts?
Answer: No, it would not.
Counter Main Question: Would the game overall be positively affected?
Answer: No, it would not.
Ethers are sadly stuck in this limbo state, at least as they are now.
Like even as a kid, the height of "just use your starter" myself & people I knew usually had a bunch of other Pokemon we used for various reasons (or we just liked) that saw usage and by Gen 2 we all generally had a full team of 6 mons we actively used
Every forum and social media post about teams almost always had a full team of 6
Just about every guidebook suggests it...the UI certainly does too....
Which in itself has sort of always made the game feel "off" when you're rocking a team of 6 Pokemon by the time you're in the second Route of the game (even if you have to double-up on a species for the moment, there's no penalty not to aside having to use an extra Poke Ball which money you'll make back in the next battle (and very likely more)). Meanwhile, all the NPC trainers you're battling at that point only have one or two Pokemon, and throughout the game you'll be lucky if the number of Pokemon other trainers have increases to 3 or 4; even against important NPCs like Gym Leaders and Villain Team Admins. Even if there's the implied sense the player is meant to be a prodigy while these other trainers are pouring their very best in the few Pokemon they're training, you're still thinking in the back of your mind "are you telling me these trainers don't have back-up Pokemon; or at the very least didn't demand both of us to battle with only a certain amount of Pokemon"? And while it makes sense that Gym Leaders would specialize in certain Types, not so most other trainers yet they do; but had they not maybe they could have more Pokemon as than you'd be put in the situation where you need to consider staying in with your active Pokemon or switching to one with a better advantage; you know, a question that may help a new player if they ever want to get into competitive.